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