Don't bother with the x850XT, too much money for too little performance in my opinion. The x800XL is a much better card for the money.
And memory depends entirely on your CPU platform. If you've got an LGA775 Intel, you're stuck with DDR2, and if you've got an AMD Athlon 64, you're limited to DDR. In my opinion, DDR is cheaper and has been proven to rival and in some cases outperform DDR2. It's more cost effective than DDR2, but DDR2 is no doubt the inveitable future.
Intel C2D E6320 / AMD Athlon X2 3800+
Gigabyte 965P DS3 / DFI nF4 Ultra-D
2GB OCZ Gold PC2-6400 / 2GB OCZ Gold PC4000
eVGA 8800GTS 320MB / eVGA 6800GS 256MB
150GB Raptor / 74GB Raptor
2x500GB / 320GB
OCZ GameXStreme 850w / OCZ StealthXStream 600w