The X800 PRO costs less than $400 now, as for the BIOS, you need to be "lucky" to get a card that is modifiable. You see, the X800 which was a candidate for overclocking became labeled as an XT, and sold for $500 each. The X800 PRO series have a mix of oc'able and not-so-good cards that didn't became XT.

I own an X800 PRO and is satisfied by its graphics quality, but when in first person shooter games, fps (frame per second) matters a lot, too. Base on many reviews and personal words from owners of the 6800 GT, they are much more pleased with their framerate than X800 PRO owners. The X800 PRO and 6800 GT costs the same price, here are some of the pros and cons from my experience:

6800 GT - Faster, with 16 pipelines, runs well on OpenGL games, uses a lot of electricity
X800 PRO - Fast on Half-Life 2 games, 12 pipelines, uses less electricity

The 6800 GT has from 10-15 better fps than the X800 PRO in general. I would recommend the GT if you're going to spend $400. But if you got more than that, try the X800 XT Platinum Edition, which said to be faster than 6800 Ultra.