Hmm....ok, so it sounds like it's not just more memory usage. But probably all of you are in sw vp mode.
Ok, try this to force it to use hw vp mode:
1) Create a shortcut that points directly to GC2DarkAvatar.exe. (Right click on the exe and choose send to desktop as shortcut)
2) Right click on the shortcut and choose properties.
3) Left click in the Target field and hit the END key on your keyboard so that the cursor is at the end of the field. Hit space. Type the word HARDWAREVP then hit OK.
Start the game and then see if it says something like this in your debug.err (with your own video card there):
Debug Message: HAL (hw vp): NVIDIA GeForce FX 5200
If it worked, continue in the game and see if you get better framerates. It might look weird because if your video card is normally starting in sw vp mode, that means that it's having to emulate something. So there may be graphics missing entirely or other weirdness if you're able to force it to run in hw vp mode.
But if that's the cause, I can have our 3D engine guru figure out something for the sw vp cards.
