I'm not very familiar with the Core2Quad's, but I would think that the CPU will be able to keep up with either of those GPU's as neither are very demanding. 4gb of ram for seems acceptable for that system as well. However, given the cheap prices of ram lately I would recommend to upgrade to 8gb.
I wouldn't consider upgrading your system to much. Better to go with a new build. If you want to upgrade graphics I'd go 7700 series or Nvidia x40,x50 series. They can be found ~ $100 and give pretty good performance.
YOu can get a new GPU and things should be better. Something like a GTX 650 or 550TI would be good. I went from a 9800 to a GTX460 and saw a great improvement. I was using a Phenom 2 triple core at the time and I do not feel like I was getting much of a CPU bottleneck. I think your quadcore is pretty similar to what I have (you may want to overclock it if possible).
However, as others have said I don't think a 640 is enough of an upgrade to be worth your while.