My second PC is an Athlon 1.4 ghz with a GF3 Ti200 (soon to be GF4 Ti4200 hand-me-down from my main rig, which just got a 6800GT). It runs Win98, and I've never felt the need to upgrade, because, well, it runs just fine. It also has Linux on it, so there's no rush to upgrade just to play Doom 3.
Of course HL2 probably won't run on Win98, and that won't run under Linux, so maybe I should just bite the bullet and upgrade the Windows soon. Decisions...
"I see a Hollow Man, gun in hand, it points my way." - The Cult