snnoopypw: Sorry, I haven't tried running 8GB on a P5B at DDR2-800. You might have to be the guinea pig and give it a try. Even if some user posts that he did it, you might try it and it might not work with your ram.

I do know that the guy that said this:

I put 4 x 1Gb PC-6400 Rambo ram on my board, and it only detects 3Gb....
was probably just running into the limitations of a 32bit OS.