All power issues I know resulted freezes or bluescreens, but never low frame rates. That would be new for me.
I must say I really do not know the exact differences between 7800 GTX and 7900 GS. Maybe the problem is that the video card with the “higher” number is not faster.
For example the 7950GT is faster than the 8600GT in most cases.
Since you wrote only that the other PC has over 100fps, tell us more about the two systems (CPU, board, RAM, HDD…)