Does Riva Tuner work with WinXP?
Yes it does. I run XP myself and it works just fine.
A few quick tweaks and turning off certain driver features such as antialiaising (or what ever its called) and V-sync can improve frame rates. It depends on what card you have, with a good card like a Ti4x00 or a FX5x00, you may only need to sharpen textures, if that. You probably won't notice much better on high end cards. You don't need a tweak program to turn off what I listed above but you do need one to sharpen textures. Through Riva, this can be done by setting the LOD (DirectX and OpenGL) slider to a negative value, such as -2. If textures and frames are good to begin with, don't bother tweaking.
BTW, although Riva advertises it works on ATI's Catalyst drivers, but I think it only works on older ones. Riva works on all Nivida Detonator drivers. There are a few other tweak utilities at Guru3d.com but I found that Riva is the easiest to use.
If you want to see how your card stacks up, visit http://www6.tomshardware.com/graphic/20 ... ts-06.html
This compares varoius cards. The site I listed uses Jedi Outcast as a test game. JO is a high frame game. The results were quite interesting. Some lower end cards preform better than high end ones, particularly when compairing Geforce to Radeon.