Well I can answer this myself now

on Win XP
1280x800 (graphics auto detect Quality as High) ~7FPS
1280x800 (Quality Low) ~9-12FPS
800x600 (Quality low but settings below shadows enabled) ~16-19FPS
640x480 (Quality low) ~30FPS
640x480 (Quality low but settings below shadows enabled) ~26FPS
@Larian (or David?) noted on KS that he was surprised it ran at all, and quite understandably felt the graphics card was the bottleneck. Also suggested using Win 7 rather than a 12 year old operating system

As I'm curious to see what, if any, difference running in Win 7 would make I'm in the process of backing stuff up before I nuke the partition and install Win 7.