I have a 2100+ XP running at 2.27. (The box I'm typing on right now) air cooled and stable.
Though my gaming rig is a 3000+ (Barton) @ 3200+ speeds and I notice a HUGE difference. (Though some of that is no doubt memory related as the 2100 is using a gig of sdram and the Barton is using a gig of 400 DDR dual channel)