Id propably get some new RAM, and possibly a new CPU, and then save the rest until something that I needed came out. Your RAM is really the only big downfall in your current system right now though.
I would recommend some higher speed ram, such as PC-3200(DDR400) or higher, and if you can get the Mushkin High Performance Black. Its about $280 for 1GB for PC-3500 @ the egg.