if i were you, i'd settle for this sorta stuff:
asrock mobo- 55.99 (am2 socket, pci express 16x)
amd dual core 5600 2.8ghz- 139.99
thermaltake 500watt power supply- 69.99
coolermaster case 120mm exhaust fan, 80mm intake fan built in- 49.99
radeon x1950 pro 512mb-128.99
and that leaves you with 55 bucks for the shipping and should there be one, the tax. some states don't have a sales tax for newegg...like florida.
anyway. that setup would MAKE it a super gaming computer. with your current RAM and hard drive, you'd have almost exactly the same setup that i'm looking at buying for myself.
here are the links, in the same order as listed:
http://www.newegg.com/Product/Product.aspx?Item=N82E16813157108
http://www.newegg.com/Product/Product.aspx?Item=N82E16819103771
http://www.newegg.com/Product/Product.aspx?Item=N82E16817153052
http://www.newegg.com/Product/Product.aspx?Item=N82E16811119068
http://www.newegg.com/Product/Product.aspx?Item=N82E16814102094
they're cheap, but very good quality parts. fairly low shipping prices for all of them too. they're all individualistic shipping. like the processor might be 6.85, or the case might be 15.99, or something like that, and then it all gets added up. some of the shipping is free for some items.
but check em out. that rig will even be good enough to handle bioshock should you choose to play it. naturally, you're not forced to take this stuff. but i think most of you will agree it's a decent list.
i'd also advise you against sli or crossfire. you're better off just buying a single good quality video card rather than buying two and causing power problems and other things. maybe if sli and crossfire were easier to use and easier on the system, we'd recommend it. but ignore sli and crossfire for now.
also. for future reference, i have a few alternative options. for example, there's another power supply that's not as good quality, but still pretty good for 49.99. it's from coolermaster, 500watts just like the other one i mentioned up there. i lost the link at the moment, but a quick newegg search will find it.
that being said, if you go with the 69.99 psu, you have 160 dollars left to play with the graphics card. with the 49.99 you have 180 to work with. (i'm accounting for shipping). if you could save up a hundred more, you could get yourself one of the best graphics chips out there. like the radeon 2900hd 512mb. 280 bucks. and then your rig would be next to unstoppable. and also very cheap. just a thought. you're so close, yet so far away from the most awesome stuff.
if i were you, i'd CONSIDER waiting and trying to save up a little bit more. then you'd have an amazing gaming computer. but if you can't get the money, or dont have the time, so be it. i say go for it.
best of luck