The CPU

The first thing to realize is that the NAMES and NUMBERS of the processers no longer have any meaning
or relationship to actual performance. I believe it's very likely that this is no accident.
Very few consumers have any real technical knowledge about computers, so they likely have not even noticed.
In order to make an informed and educated choice, you must consult CPU benchmark charts and compare the prices.
If you plan to overclock your CPU, be advised that not all chips will overclock. Even if a chip is known
to be a good overclocking candidate, not all chips will overclock as well as the ones you readabout.
It's the luck of the draw on how well it will overclock.
The relative performance of different chips also depends on the software your running.
It's not at all uncommon for a much less expensive chip to beat an expensive one.
The Intel Extreme chips costing double or triple the cost of standard chips only offer 5-10% more performance.
The older Intel chips cost more than the newer ones and have much lower performance. Intel prices do not fall over time.
The AMD chips with 512KB cache have very poor performamce in games.
The AMD BLACK editions do not overclock that much better than the standard chips.
The 65 nm process chips are energy efficient and not for a gamer

A great web site for benchmark info is TOM'S HARDWARE GUIDE
CPU selection depends on what you plan on using the rig for. Best to do your homework, look at the benchmarks,
compare prices to benchmarks for the applications you plan to run.
Find out before you buy it if it's a good overclocker.
Games are the cutting edge of software, they will be the first to optimize code for dual and quad core chips.

The Main Board

Motherboard selection can be critical. Choose wrong and your system will have unexpected limits.
Not all boards are created equal. There are certain facts to keep in mind.
On board video = slow system memory and poor video performance even if you don't use it.
One way to make an ultra cheap computer is to put everything on the motherboard. That's what
you find in cheap desk tops. OEM boards, even OEM versions of good boards are limited by the
components used.
Not all motherboards permit overclocking. If you plan to overclock, do your homework.
Best to buy a proven performer that has a track record, online reviews, owner feedback, etc.
Compare costs, in some circumstances it may be cheaper to get a higher performance chip from
start instead of a cheaper chip plus an expensive cooler.
SLI and crossfire boards are for high end use. Do not buy one if you plan to install low or mid range
cards on it. You will just be waisting your money.

The VIDEO CARD

There are so many different video cards to choose from, so many really cool names, hot looking retail boxes,
but only 2 chip makes to choose from, NVIDIA and ATI. That's right, over a dozen brands and many dozens of
different versions of the chips in several hundred cards to select from. Lots of homework to do in order to
make a good choice in what exact card to use.
First choose the chip maker. If your going to run LINUX, choose NVIDIA because of the driver support.
If the box does not list the technical specs, you can assume that's because they suck.
A really cool looking box means nothing, it's the numbers that count.
Now look at the chip benchmarks and actual specs.

What to look at;
GPU number - GPU generation and version
Clock speed - how fast
Memory speed - how fast
Memory type - DDR, DDR2, DDR3, and DDR4 memory
Memory - 64 bit vs 128 bit vs 256 bit vs 320 bit, 384 bit vs 512 bit memory
Pixel pipelines - 8, 16, 24, or more
Stream Processors - 32, 96, 128, 320
Memory amount - 128mb, 256mb, 320mb, 512mb, 640mb, 768mb, and 1Gb memory

You don't have to be an expert to make an informed decision on what to buy.
Again, the latest, greatest, and fastest will be way expensive.
When you look at cards and prices there will be an obvious price differences
between GPU generations and versions. Many versions of the same chip have different specs.
Different clock speeds, memory speeds, memory bandwidths, numbers of pixel pipelines.
Fancy names are given to seperate the versions, GT, GS, GTX, XT, extreme, LS, LT, etc.
You have to look at the actual specs to compare performances and to know what the card is.
Whenever a new generation comes out, the previous ones prices plummet. You best deal may be available
some time after a new model makes it's debut. Again, last years hot item may represent the best
and biggest bang for your buck available.

The CASE

The case you choose may not seem a critical choice, but if you choose poorly,
it will limit the performance of you system. A small poorly ventilated case
is a poor choice for a gaming rig. If you plan to overclock, you will need at
least a mid tower size case in order to fit a high performance heat sink and fan setup.
Case height and width needs to be addressed. A case with the power supply mounted on the
side of the motherboard in order to get a lower case height is a poor choice.
The power supply typically blocks the stock heat sink and fan in these cases making
them less efficent. No high capacity, high performance cooling can be added in these cases.
Heat sink and fans have gotten larger, taller, and require a larger wider case.
There are lots of cases with pretty lights and cool graphics, but are they wide
enough to install the cooler you need? Can you fit the very large performce video cards in it?

What is the case made of?
Aluminum - Best for thermal and lightest weight
Painted metal - Cheap cases have very thin metal and are easlily deformed
Coated metal - Quiet, but bad for thermals
Plastic - Very bad thermals, but very quiet

All in all, Aluminum is best and bigger is better, a full size Mid-Tower case is a must for a decent rig.
A quality full size ATX case will last until ATX motherboards are gone. The case will outlast many motherboards
power supplies, and the rest of the internals. Think of it as long term infrastructure.

The Power Supply

You need at least a 750 watt power supply to make sure you will have enough power.
Power supplies vary in price and quality. A cheap power supply may be OK for the kids
computer that only does word processing and surfs the internet. For a good rig,
you need to choose a good power supply. Bigger is better when it comes to power supplies.

Things to look for;
Cables and plugs, how many, how long, and for what?
Is it SLI Certified?
Does it have active PFC?
Does it have plugs for SLI (2 video cards)?
Fan configuration - 140mm, 120mm, 2 x 80mm, 3 x 80mm, 1 x 80mm?
MTBF - Mean time between failures - get one with 100,000 hours or better

Modular type cables and connections have both pros and cons. The pros are easier cable routing
with fewer cables, improved airflow, and better astetics. The cons are more connections mean less reliability
and the possibility of losing the unused cables in your office's black hole.
I prefer a supply with a 120mm fan, they are typically much quieter and still push out the heat.

Memory

I use value ram and it works. I have never attempted to Over Clock memory.
If you want to overclock, you will need to buy better memory.
So, all I will say about memory is I recommend you buy a brand with a lifetime
no questions asked warrenty. It will hold it's value better for when it come time
to sell it on ebay after your next generation gaming rig project.