Its dual channel memory. Use 2 sticks or 4, not 3 or you will reduce performance. 4GB is all you will need for gaming anyway.
Either CPU should be good. There are a few games like GTA4 that will play better with more cores, but most still play faster on a faster dual core.
I dont know if you budget would allow it, but ATI is out with the 5800 series GPUs now. If you want to use DX11 in a couple years when it becomes common then spending an extra $100 to get a 5850 (which is also 60% faster than a 4870) might be a good idea.