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.
You may want to check how much better the 640 is vs the 9800. According to Tom's Hierarchy Charts, it would be a pretty lateral "upgrade". Maybe you want to consider a 650 to make it more worthwhile? That may start to chip at the CPU though. Link - http://www.tomshardware.com/reviews/gaming-graphics-card-review,3107-7.html
If you did want to upgrade the ram, you would need to make sure you were running a 64-bit version of windows to take advantage of the extra ram, as 32-bit windows only sees up to 4gb of ram.