Video card is a lot more important.
X1950XT with 4000+ will totally kill a 4600+ with 7600GT. No contest.
The 4600+ is a better CPU because it's a dual core, but if your dad wants to game good, then the video card should be the first priority.
Try looking for a 3800+, or a Core 2 Duo E6300, and a X1950Pro. That would be a very nice setup, and the CPU will be ready for future, multi-threaded games.