Well the latest version of nVidia ntunes adds the overclocking tools to your nVidia control panel and works on XP, Vista 32 or 64bits.
Version: 5.05.38.00
Release Date: April 26, 2007
Supported OSes:
Windows XP (32-bit),
Windows XP x64 Edition
Windows Vista (32-bit)
Windows Vista x64 Edition
More info and download:
http://www.nvidia.com/object/ntune_5.05.38.00.html
Works even if you don't have a nForce chipset and let you overclock your GPU on any motherboard.
I just installed ntunes on XP to try OC my 8600GTS and will install it on Vista too. Seems working fine in XP so far.