I'd also love to help test it, but I'm useless if you only need Vista testers. I have one system with a Radeon 9800 and a cardboard box with a GeForce 3 (Athlon 64 and Athlon XP, respectively), if you're interested in testing low- to middle-end (?) XP systems. I can also test it against Wine (under 32- and 64-bit Ubuntu) if you're really interested, but that might not be a good idea since the bugs could also be inside Wine itself.
Oehr: Have you tried (if possible) really old drivers? On my Radeon 9800, for example, anything newer than the 7.11 drivers (which are over a year old) borks Aquaria's textures. (And that happens to be when ATI did a major rework of their OpenGL code, I believe.)