The subject system includes:
P4 Prescott 3.0Ghz
Chaintech Summit MB
1024MB Corsair XMS DDR3200
Creative Soundblaster Live
2 80mm fans
Sony DRU-710A DVD Burner Thermaltake 430W PSU
Would that setup have enough power to run a 7800GS instead of the 6600GT? I realize the PSU only has 18A on the 12V rail but I've read that the 7800GS only draws about 30W more than a 6600GT. If I remember correctly, the extreme PSU calculator puts the system at around 280W at peak usage, but this doesn't specify rail amperages. I found an eVGA 7800GS for $265 AR and think it's time for an upgrade, but don't want to have to upgrade the PSU too.
I have a 3.0 P4 Northwood w/2gig RAM, one 160 GB HD, one Optical Drive, one floppy, and an Antec 430 Truepower PS in an Antec Sonata case with an additional 120mm fan. The Antec 430 powers my system with the BFG 7800 GS with no glitches whatsoever - smooth, cool, and error free!
Problem is, that Thermaltake 430's 12V is less than an Antec Smart 350W has on the 12V. The current Antec Truepower II 430W has dual 17 amp 12volt rails compared to that thermaltakes single 18 amp rail. I bought that same Thermaltake cheap from newegg and didn't use it in a build because the 12V was so rated too low. It might work well for him, it might not.
Finally, some input regarding the 12V rail. That has been my concern from the beginning. 18A is quite low for a 430W. Unfortunately, I was uninformed regarding the different PSU rails when I purchased it a year ago.
You can always try it. If it's not stable, you know why and what has to be done to fix it. But Even if by chance you have enough power, that PSU will be giving all shes got. I'd get a beefier one if it were my system.