Are you actually having a problem with video?
If not, there may be some glitch in how your System Profiler displays hardware, and not a real "missing" vram.
If you boot your MacPro to disk one of your original restore DVDs (the set of 2 grey ones that came with your MacPro) you can run the Hardware Test that's on that disk. Boot to that disk while holding the D key (for Diagnostics, eh?), and you can run the extended test. If you have a lot of RAM, the hardware test will take much longer.
Did you reinstall your OS X 10.6 system yet? The glitch in hardware reporting may be helped after you do that...