Generally AMD based motherboards are the best for gaming. You have to choose between a duel core CPU or single core. AS of right now, there are almost no games that support duel core technology. However this will change in the future. Furthermore, your motherboard may require a BIOS update if you buy a duel core CPU. Read the article on the home page of techspot regarding "build a high performance PC." If you elect to go with single core- the AMD FX57 is the best chip on the market for gaming.
Intel and AMD motherboards are NOT interchangable. Each has advantages and drawbacks. Read this:
https://www.techspot.com/articles/highendpc/
I have had really good performance with MSI based motherboards. I have owned two of them. I do not recommend ASUS although others have had good experiences. Gigabyte seems to be another recommended model. Look for the features that you need in all of them.
Bear in mind most gamers will want to take advantage of the new SLI technology offered in the newest PCI-E motherboards. You can hook up two PCI-E video cards in SLI format on the same motherboard to double your graphics processing. Most games are video intensive. SLI, however, is expensive and be prepared to lay out some bread.
NVIDIA seems to be the better of the two major graphics chips manufactuers. I have used both ATI and NVIDIA and ATI has had a lot of buggy cards.
Ensure your computer has a very large wattage power supply. 550 watts or more. Go to Tom's hardware and read their articles on power supplies. Price does not equal performance.
Ensure for gaming you have 1 GB of ram. I recommend Crucial, micron, or corsair. Buy memory in matched pairs from the same manufactuer. DO NOT mix brands of memory - it is a bad idea - you'll usually wind up with problems.
For hard drives, 40GB is sufficient for most people. However, you may want to go larger if you have videos and music. Always have two hard drives. A primary and a backup. Go with SATA or if you want to save a few bucks, IDE/ATA format. Do not however, mix both. Windows is fussy about both on the same system. Use one type or the other. Although SATA is faster than the older IDE/ATA format, the performance is not really noticable. Burst speed advertised on hard drives is advertising hype. What counts is SUSTAINED transfer rates.
As far as video cards go - try a PCI-E card in SLI mode. The 7800s are the best out there now. But video cards tend to change quickly. You'll want ample cooling in any gaming computer. Ensure your case has at least 3 to 4 fans. Two for induction and two for exhausting.
now if this cost seems prohibitive, remember - you're future proofing your system. If you want a budget system, you can build one for $500. Tom's Hardware Guide has an article on what to get. But remember, older components will become obsolete quicker.
http://www.tomshardware.com/howto/20051014/the_500_gaming_machine-02.html
I have an Athlon XP based system that runs games quickly and nicely. I play Doom3, Return to castle wolfenstein, and medal of honor. It's an 3200+XP CPU on an MSIK7N2 Delta 2 platinum motherboard. I have an NVDIA FX5500 graphics card.
At the moment I don't really see the need to spend $$ on duel core CPUs, I just don't see enough software out there that supports it, nor do I see the value in 64bit CPUs, like the Athlon64s - for the same reason. But if you want to get a CPU that's modern and will support 64bit applications in the future and you want gaming- go with either an Athlon 64X2 4800+ or an Athlon FX57.