I’ve finally finished upgrading my system (I dropped in an ATI 9800 Pro [which detects as an XT and clocks accordingly] and an Athlon XP 3000+, but needed a beefier heatsink), and although my CPU is underclocked a bit (my old motherboard only goes to 12.5x multiplier, not 13x). My Doom 3 timedemos have gone from a rather anemic 20fps at 800×600 Low Quality to a respectable 45fps at 1024×768 High Quality.
That being said, I think I’m pretty much over this “installing my own hardware thing.” I think I realized this after I managed to pry off the old heat sink and as I was removing glue residue from the new one.