No... don't change the mobo, I thought that was the mobo that drunky recommended to you... you want to stay with PCI-e slots. PCI-e is the "newer" technology. You can still get a good PCI-e 6600GT vid card with the money you have left over.
This card is not going to be the best performer in these games, but I would normally suggest a 6800GT for someone that wants the performance that you seek, but it is just out of your range. Check out the review that I gave above.