Celeron 1.2GHZ & WinXP SP-2..
I am acctually at the mo, using a PIII 450Mhz, 384MB Ram and it runs like a dream for casual stuff.
Obviously i don't run games (bar 3D Mahjonng) or DVD movies (ain't got a DVD Drive :P ) but it runs XP SP2, OpenOffice 2.0 RC, Music, Firefox and networking stuff...