Thanks so much for chiming in, both of you! As it turns out, the integrated gpu must've been getting in the way. I booted into bios without the 6870 installed. I disabled the integrated gpu completely (kind've scary because the screen went black right after), shut down, reseated the 6870, and viola it booted up great.
Also, I started windows and downloaded the latest ATI drivers before I did all that, so that they were already there when I installed the 6870.