Advice on building a better gaming pc

With the release of crysis warhead, my humble Nvidia 6600 has to go I think. I'm just wondering would be worth me upgrading my CPU to dual core? It's an expensive upgrade because me Motherboard and ram would need to go. At the moment it's a athlon 64bit with 1GB DDR1 ram. All this powered by a 480W PSU. I'll also need vista since DX10 is only on vista. I want to reuse as much as possible! I'm already buying a new LCD TV!!! I've just started work so am not rich!

I'm assuming you have a socket 939 board. What CPU do you have exactly? I think you would be fine if you added an extra 1GB of RAM and a Radeon 4670, assuming you have a PCI-E slot. If you have at least an Athlon 3700 you can overclock then you can probably play crysis warhead with medium settings, altho you may have to lower the more CPU intensive settings to low especially for the heavier parts of the game.