My old Athlon XP 2400+ doesn't overclock very well at all (T-bred, not Barton mobile.. those will!). I think I managed 300MHz more, but I also needed 0.2v more to gain that (1.85v)
Oh and I don't bother with Prime95 anymore. Takes way too long to fail. Try OCCT... far better!
http://www.ocbase.com/OpenBeta/OCCTv0.91.exe