You will also probably hear a lot of advice here to hold off for another couple of months since there are some new lines being launched. Prices should drop soon after. Also, HCC GPU is still in BETA and it would be wise to wait to see what GPU to invest in. You would also be wise to explain if you are focusing more on GPU or in an all around system. Motherboards will differ greatly and if you want multiple GPU's in one system, the suggestions will very greatly as well. Probably most important decisions is cooling solution and power supply. Let us know what you are pushing for, because a heavy CPU crunching box design can greatly differ from a GPU focused box.

Edit: Also, what will be your focus? Specific project, points, badges? Are you looking to support outside of WCG as well?

I went to Micro-Center and bought the parts, cpu, motherboard and ram for $400.00 and built myself an AMD 6 core screamer. I already had a case, power supply, gpu etc, etc as I was upgrading fro ma dual core to the new hardware, so it was cheaper for me. If you don't want to go that route you can buy a nice quad core pc for a similar price but it won't take the latest gpu for crunching, that would require a power supply upgrade too. Power supplies can be $75 to $100, or more, US! It sort of depends on your tech skills and the amount of money you can spend. If you are strictly buying a new pc then a gamer pc should do the job nicely which would include a gpu and the appropriate power supply.

I got one of the best I could afford when adding it into the price of my new system. I also put it together myself, which isn't all that hard these days as long as you're careful about stuff, and do a test fit of the cpu fan prior to pasting. :)

Another thing to remember, often it's way easier to put the cpu, ram, and cpu fan/heatsink prior to putting the motherboard in the case. It's much easier to get at it all that way. Heck, with some cpu fans you HAVE to do it this way, as you attach something on the back side of the m/b.