it is likely that the 6200 is bottlenecking the 8800
try a more powerful gfx card
I think my options are limited with using the PCI slot to power the extra monitor, which is why I ended up with the 6200. I also wanted to stick with Nvidia for both cards to avoid driver issues.
Maybe i just need to go with a video switcher of some sort to toggle between the 2nd monitor and the Cintiq, but it would be nice to get the 6200 to work.