Your GPU is already faster than your CPU. Your next move should be a new cpu board and ram.
Just for fun I did some 3dmark 06 tests on some of my old computers a while back.
I have a core2duo 2.18. It scored a 9000 with a 8800GT, it scored the same 9,000 with a gts250 and then the same 9000 with a gtx470. The point here is the cpu board or ram were the slowest part of the computer which prevents the gpu from really stretching its legs. I then droped a core2duo 2.66 in it and the score jumped up to 11,000 for the 8800gt, 12000 for the gts 250 and around 13,000 for the gtx470.
I then did the same test use the stalker benchmark. Again all 3 video cards got about the same score. on the 2.18 and a little bit of a lift from the 2.66.
Point is a new cpu is the way to go but spending money on an outdated 775 chip will only give you a little bit of a lift. Better to go i3-i5-i7.
I would do a i5 for around 200 a new board for around 100 and ram for around 100. Then in a year or so get a new gpu.