The 7900GT problem was addressed by Nvidia long ago. I doubt its his problem.
It had to do with OC'ed 7900GT's. The problem was fixed a while ago.
1 difference is your 20" proly uses a much bigger resolution than your GF's 17".
Who is hosting the LAN? Usually the host gets the best performance.
Is it smooth playing online? Whats your ping? How many FPS are you getting?
Do you have a power connecter connected to the front of your GPU?
Vista? Also, what would the C++ error have to do with this?