I'd say balls to the wall and just do an entire makeover.
Hercules is spot on with the gfx cards.
In gaming pc's, AMD seems to be the preferred cpu for gaming(though my intel works fine; it may be different for high end games, indeed i've never had an amd -or- a high end game
I think ati's, even though their top-line card is supposedly
able to outperform the 7800 gt (not sure on this, only heard opinions; help here, someone?), but everyone seems to like the 7800 gtx that has it, or if you're on a tight budget, overclock a 6800gt to about the same level, if you go with a system that could take it