An update:
My ebay power supply arrived on Saturday.

I successfuly upgraded from 6800 to 7800 with only one snag:
Following the instructions on the first post, I uninstalled and cleaned my old drivers before swapping cards... however, when I booted into windows using the 7800, the screen was garbled (it worked fine as it was booting up, and in the bios menu, so I knew it was a windows driver issue). I solved the problem by booting into windows using VGA mode (a safe mode option), which worked!! (safe mode did not work, so I tried the vga mode).
In VGA mode I was able to re-install the drivers and got the card working normally.
I then proceeded to OC using the other boot disk posted in the forums. I just chose what seems to be a stable OC for most people (440/1200) and left it alone after a bit of testing, since it seems to work fine.
stats in sig!