Its going to be expensive to upgrade to an Nforce4 mobo now. If you go to NF4, you will have to get a new vid card anyways. NF4 mobos have PCI-E graphics slots, while your vid card is an AGP.
What I recommend doing (I did it myself, actually), is getting an Nforce3 Motherboard + skt939 CPU, and keeping your vid card for now. Then, when you can, get a 6800GT AGP, or whatever else you like.
This option will give you a nice upgrade, but will allow easy improvements in the future. AGP is going to be around for at least a little while yet, so you wont be selling yourself short