Go for 8GB of RAM and get 2x4GB or if you want 4GB go 2x2GB for best performance. 1x4GB will only give you single-channel performance. 2x2GB will give you dual-channel performance.
Using 2x 40gb 7200rpm hdd's in raid to save on money. I think I am going to go with the FX-6100 it scores close to some of the I5's on processor benchmarks.