I personally don't shop in the US, therefore I really can't advise which e-commerce is the best. Make a search through google and try finding the best service/price available.
That being said, I'll base my post on price estimate off several websites (including rebates atm).
Casing:
Antec Nine Hundred - 118$
Power Supply:
OCZ GameXStream 700-Watt - 99$
Motherboard:
Gigabyte 965G-DS3 S775 - 139.99$
Memory RAM:
Buffalo Firestix Dual Channel 2GB - 109.99$$
Processor:
Intel Core2 Duo E4400 2.0GHz / 2MB Cache - 149.99$
Video Card:
EVGA GeForce 8800 GTS Superclocked / 320MB - 284.99$
Hard Drive:
Western Digital / Caviar SE16 / 500GB - 129.99$
DVD Writer:
Pioneer DVR-112DBK - 39.99$
CPU Heatsink:
Zalman CNPS9700NT - 69.99$
Total (without shipping+taxes): 1141.93$
I haven't verified the default voltage of the Buffalo Firestix and hopefully it is set at 1.8V, otherwise it won't start with the Gigabyte DS3.
Not necessary:
Physics card: BFG Ageia PhysX - 149.99$
Sound card: Creative Labs Sound Blaster X-Fi XtremeGamer - 94.99$
Total (without shipping+taxes): 1386.91$
That's my build. Although if I were you, I would wait.
P.S.: Feel free to ask me what or why I choose an item.