Your proposed system does not require a lot of power on the 12v rails (which is the most important). Assume the following system:
E6400 OC'ed @ 3.2GHz.................................73w
CPU power consumption
7900GT.......................................................49w
GPU power consumption
2 320GB Hard Drives...................................32w
2 DVD Rom Drives......................................32w
2 120mm Case Fans....................................12w
Total Estimated Power on 12v rail................198w
Total Amps on 12v rail................................16.5a
The above assumes every single component is working at 100%. That is very unlikely. In fact, it's nearly impossible. When playing a game the above system will like draw 170w or less. That translates to about 14.2 amps.
All other components like RAM, motherboard, sound card draws power from the 3.3v or 5v rails.
A powerful PSU is not necessary to power the system above. The ENERMAX EG301P-VB-1-FM ATX12V 300W Power Supply is more than enough.
http://www.newegg.com/Product/Product.asp?Item=N82E16817103463
This PSU provides up to 22 amps on the 12v rail, or 264w.
Of course if you know you will be dropping in a 8800GTS later on which uses in excess of 125w, then you should get a more powerful PSU like a 400w PSU. Or more depending on how many additional hard drives will be added in the future.
The best time to buy the PSU is when you have the $$$.