good luck in finding a good Motherboard and a AMD 64 CPU and a good Video card for 400 bucks lol
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$400 will be a little tight man, depending on how good of a system you want. You have to decide if you'd rather have a weaker CPU or graphics card so you could determine what to spend more on.
You should have started with the mobo, cpu, vid card, and RAM as your first components THEN start buying everything else. You kinda went about it bass-ackwards on this one.
The current good socket 939 boards from good companies like ABit and MSI are around $125, the cheapest Socket 939 CPU I believe is the AMD64 3000+ which is $150 so there's $275 right there and you now have $125 for a vid card which isn't going to get you much.
A 'DECENT' card would be something like the 9600XT 256mb for $150 but even that isn't great and you won't really be able to play the latest games out with the that great of settings.
AMD64's are overrated right now. There's no need in getting a 64bit CPU when we're working in a 32bit environment. If you get a Socket A motherboard with a good Socket A overclockable chip you can get way better performance for a lot cheaper price, BUT of course it requires a little overclocking skills, but not much really if you're even slightly interested in learning.
Basically you'll only be able to afford the cheapest CPU in this scenario which in the big scheme of things, is crap considering it won't be fully utilized until around 2006.