Well it depends how patient you are. Personally, I'd wait the extra month and a half or whatever for ATi's HD 5 series cards to come out. They should kill any nVidia on the market for a good price. If you have to buy now, Radeon HD 4890's are a pretty good deal. Crossfire seems to scale better than SLI, and ATI works harder at it, although SLI has slightly broader developer support.
For CPU at this point I think the gaming ability, value, and easy OCability of the Phenom II black edition puts it ahead of the Intel i7, which seems to me to be overpriced, at about $80 more. It I was encoding video all day, I might get an i7. In this case, that money is better put to graphics cards.
So, the list:
Motherboard: MSI 790FX AM3 Crossfire DDR3 motherboard, about as futureproof as a motherboard can be these days. It does limit you to only crossfire as a multi-gpu solution, though single nVidia cards will work. An X58 i7 board will cost another $100, which is enough to trash this board and switch to an nVidia Am3 board in the future if you want, though I doubt you will. The AMD 770 board the other dude suggested leaves you stuck with 1 graphics card. Better to leave the option of 2, 3, or 4 if quadfire or physX ever get developed.
$165
http://www.newegg.com/Product/Product.aspx?Item=N82E16813130223&cm_re=msi_790fx-_-13-130-223-_-Product
Processor: Phenom II 955 Black Edition 3.2ghz 45nm Quad. Overclockable to 3.6-3.8ghz with extreme ease.
$190 (I think the 20$ more than the 940 is well worth it)
http://www.newegg.com/Product/Product.aspx?Item=N82E16819103674&cm_re=955_phenom-_-19-103-674-_-Product
Memory: 4gb DDR3-1600. About the same cost as DDR3-1066, but obviously way faster. 4gb was sufficient on vista, and 7 isn't quite such a system hog.
$85
http://www.newegg.com/Product/Product.aspx?Item=N82E16820231277&cm_re=4gb_ddr3-1600-_-20-231-277-_-Product
Hard Drivers: RAIDed dual 1tb hitatchi 7200rpm drivers
$160 for two
http://www.newegg.com/Product/Product.aspx?Item=N82E16822145233&cm_re=1tb_hitatchi-_-22-145-233-_-Product
Power Supply: 850W Corsair (great PSU maker) supply, future-proof for crossfire, bla bla, awesome, good investment to take with you down the road.
$120
CPU Cooler: Zalman CNPS 9700 will make sure the CPU stays cool and quiet even under overclock. Did wonders for my Phenom 9850BE.
$55
http://www.newegg.com/Product/Product.aspx?Item=N82E16835118019&cm_re=CNPS9700-_-35-118-019-_-Product
Case: Antec Sonata? Looks like a str8 foraward case, decent price. Personally I just built in my moms '98 ATX case and left the panels open. Really. Then again, I'm a bit of a cheap skate.
$80
http://www.newegg.com/Product/ProductReview.aspx?Item=N82E16811129057
DVD-RW: LG Burner, I like mine, but pick whatever you want
$30
http://www.newegg.com/Product/Product.aspx?Item=N82E16827136167&cm_re=LG_DVd_burner-_-27-136-167-_-Product
Okay, so here we are $880 + shipping (20$, since case shipping is free)
Now you decide: Do I want to buy video cards now, or wait for the cheaper better Radeon HD 5 series in a month or two.
If you want to buy now, I'd suggest:
2 Radeon Hd 4890's (O.C. versions really kick ***):
http://www.newegg.com/Product/Product.aspx?Item=N82E16814131167&cm_re=hd_4890-_-14-131-167-_-Product
About another $400
If you just want a holdover until the HD 5 series, I would actually recommend a (one) HD 4890 OC. The top of the line cards hold their value really well, so you can probably ebay it off when you're done with it (change to HD 5870 crossfire or something) and take minimal losses. Or you can buy two.
Mouse: Microsoft sidewinder, great gaming mouse, I love mine.
Operating System: Windows 2000 pro, stick to the classics :P
Monitor: Samsung, Dell, or Gateway 1920x1200 widescreen. If your buying a primo gaming machine, you might as well make it work!
Not quite sure how the other dude calculated the cost of his system, seems like he made a mistake in his math. I'm getting 4890 envy just reading this. At least 4850's still get it done and get it done good. BTW, Crossfire scales well, even in Crysis. I would know
REALLY IMPORTANT, IF YOU LIVE IN CALIFORNIA, BUY FROM TIGER DIRECT INSTEAD OF NEWEGG TO AVOID SALES TAX. WE'RE TALKING $150 HERE. YOU'LL NEED TO RE-EVALUATE A FEW BRANDS AND MODELS TO FIND GOOD REBATES/PRICES (PSU, RAM, VIDEO CARDS)