I initially booted from the stick with my 6870 in the slot, a white screen appeared and I started to reset the rig. it would turn white when I was installing 10.8 with the card still in, so I decided that it's going to hang on install and I need to take it out; turns out that half a sec before I pushed the button it nearly switched to the installer screen -- I only understood that after I took it out and rebooted and got the same screen. It hung on the grey apple logo screen for a while, even the cursor appeared, and after 30 seconds it switched to the installer. right now everything works fine, updated without a hitch, plugged the GPU back in and it works like a charm.
TL;DR: you probably won't have to swap out.
i5-2500K, Gigabyte Z68X-UD3H-B3, Gigabyte 6870.