I have just installed a Gainward GeForce 2 Pro 450 and the Open GL performance seems to be exactly the same as the TNT2 ultra card that I took out.I am specifically using it to accelerate Lightscape.
The Direct 3d performance has increased so I assume the card is not faulty.
How much of a performance gain should I expect between the two cards using Open GL?
I was expecting a huge difference…!!! 
Fraser
This highly depends on what the software bottleneck is. If the bottleneck wasn’t OpenGL, you won’t see a difference.
Also, if you have a very fast CPU, you may not be getting a huge benefit from the card’s T&L.
Things where you might see a difference: you should be able to up the resolution without losing speed, and you might see a difference if you have a lot of polygons on screen.
I don’t have any experience with Lightscape, but I know that with low end CPUs (under 500MHz) there’s quite a difference between TNT2 and GeForce in 3DS MAX and Maya.