Woohoo, I finally got around to getting a replacement power supply for my XP box, and things are finally back to normal — I no longer have to underclock the CPU by 25% and I can take the GeForce 2 out and put the Radeon 9700 Pro back in.
Those two factors alone had been making quite a difference. After the power supply started deteriorating, I had to cut the resolution of a lot of games down to 800×600 and had to go from having every advanced graphics option in World of Warcraft turned on, to turning almost all of them off just to make it playable. I installed GTR while it was crippled and was getting 15-25 FPS, but after restoring it, it shot up to 90-100+ FPS, enabling me to turn on higher levels of detail.
I keep getting tempted to put together a nice, new Athlon 64 / PCI-Express system, but now that I’ve got this one fixed up again, I think it’ll last me at least another year or two.