I'd recommend the ATI 4650 they are only $85 have DirectX 10, buy the Sapphire brand one they are trouble free. If you want a Directx11 card check out the ATI 5750 (just a few more shaders etc. than the 4650). They are currently around $160, but they may drop more in price with the 6000 due out soon.
Faster memory won't do anything noticeable except for a handful of software.
Most games are still only work on a single core so an extra +600mhz or more CPU would make a difference, but it depends if the game is CPU intensive.
