GPU

Desktop GPU performance comparison table

NVIDIA
The last driver version to support XP is 368.81 (x86, x64), or 355.60 (x86, x64) for the GTX TITAN Z.

It's possible to modify the 368.81 driver to add support for later 9xx series of GPUs, as well as the GTX TITAN X.

Therefore, without driver modification, the most powerful GPUs usable on XP are: GTX TITAN Z, GTX TITAN Black, and GTX 780 Ti, in this order. (TITAN GPUs are professional-grade, so ordinary users might not want to pay such high price.)

With driver modification, the GTX 980 Ti and the GTX TITAN X can be used. These are more powerful than the GTX TITAN Black, but less powerful than the GTX TITAN Z.

AMD
The last AMD GPU to support XP is the HD 7990.