Windows XP 64 Bit Edition

Is there a special version of opengl that I can install for 64bit windows?

No, because opengl is not a piece of software that you install.
It is implemented in the drivers.

Select Graphics Driver
Select Geforce and TNT2
look at th very bottom of the third column

press OK, follow instructions. And then all your GL and D3D games will run better.

Will it really improve performance on a GFFX 5950 Ultra? I’ve tried most of the 6x.xx drivers and they all SLOWED my performance drastically- the best I’ve been able to find are vers. 56.55 and 56.72

Last time I checked Win64 was still beta, to expect fast and stable drivers for an operating system that is still progressing and may or may not change part of its internal behavior from build to build, is quiet a lot to ask for.