I'm seeing better framerates with my GF2 at 1280x1024 on an Athlon 1.4ghz, so I wouldn't immediately toss out the vid card. The cpu is probably one area that is holding you back though. You might try tossing out the motherboard and cpu and going for some more muscle. An intel 845 mobo with a new P4 and some DDR SDRAM might be a good thing.
Or you could drop some coin on an athlon mobo ($80-$150) and an athlon XP 2000+ ($93) and 256 meg DDR ($70-$90). If you do this though, you may want to ditch the Soundblaster Live since there used to be issues over PCI bus utilization with soundblaster and VIA chipset motherboards, and even though they're "fixed" my understanding is that the fix slows the system down somewhat. I could be wrong about that, but you could even get a new mobo with 5.1 surround sound onboard and get better sound without even needing a sound card. Soon-to-be-released NForce2 motherboards ought to be great in this capacity as they have very nice onboard sound and ethernet.
I hate to say it, but RDRAM was a gamble to begin with and it doesn't seem to have paid off very well for Intel. Low latency helps games more than maximum memory throughput, and even generic SDRAM has better latency than RDRAM. Whatever you do, don't buy more of the PC 800 RDRAM.
Also, make darn sure that you aren't running any background diagnostic software. That stuff is only needed if you're 90 years old and don't have a great-grandchild to keep your computer healthy.
