Rather say how much money you have to spend, what components you can keep - eg RAM is DDR2 and can be kept in a newer board etc....
r6400 can be improved upon for sure, the 8000 series, the quad 6600 , the quad 9000 series. Most of these though need a newer board.
If you have the dualvsta due to keeping ddr RAM and AGP gfx, then you will be limited to a cpu that has a 1066 fsb I think.