This could also be a chipset limitation.
I don't believe you mentioned your motherboard's complete model name,
but you did mention it was an Abit AV8. This means your motherboard's
chipset is VIA's K8T800, which really pales in comparison with
nVidia's nForce4 variants in terms of overclocking performance.
The chipset is one of those factors that almost never gets considered
in an overclocking exercise.
As an aside, I'd like to caution you: There's a very good chance that
you may corrupt your WinXP installation if you haven't locked your PCI
bus @ 33MHz. Make sure that you enable PCI bus lock in your BIOS.
Failing to do so would most likely speed up your SATA and IDE channels
as well, which will corrupt your boot sector.
The same thing has happened to me, with VIA's more advanced K8T890
chipset. I neglected to lock down the PCI bus, and it corrupted my
father's WinXP installation.
Hope this helps!
>> Stay informed about: Overclocking problem with AMD Opteron 165