I'm in the same boat you're in, except I don't have the luxury of RDram, I'm just gonna wait til the Athlon 64 prices go below 200 bucks, then get a new system. I can't OC either, try DL'n the Intel Application Accelerator, I did, and I actually noted a difference in overall performance, not a huge gain, but better than it was. Try it out
HERE I hope this helps ya a bit.

Oh yea, there are SOOOOOO many utilities available at
www.pcworld.com under downloads that can help you regain performance little by little, we're talking registry cleaners, spybot removers, ram governers, fan/heat monitoring programs, they're all worth a looksie.