u wont see a big difference if u change ram. ddr3 is good if u have new platform (intel x58/p55/p67, amd 890/880/870) and newer cpu (i3/i5/i7 or phenom II). but 4GB ddr2 800Mhz is also fine. just change cpu. keep your money for future upgrades.
look at this chart: http://www.tomshardware.com/reviews/phenom-x4-955,2278-10.html
the RAM difference in gaming is very small.