need help to upgrade pc

i am looking to upgrade my pc to an entery level gaming cpu.
i have : intel pentium(R)D 2.66Ghz
assus P5P800-VM
256 MB RAM DDR.

i need to upgrade to:

2GB DDR2
512MB NVIDIA
320/250 GB SATA

i should change my processer or its ok if not which one should i get AMD/Intel??....pls pls help me