Well, not totally black. There is some incoherent white pixellation at the very top of the screen.
OK, this is what happened:
I swapped out the nvidia 8800 GTS card that's been in there for way too long for a Radeon 6870. It freaked out, as I expected, after booting up again, saying it couldn't find the old card and gave me an extremely basic-looking screen with low res and none of the effects. No biggie, I expected this.
I uninstalled all the nvidia components from my system in Synaptic, and gave another reboot. That's where the fun started.
After the reboot, I got a horrible looking screen; very low res, colors are completely off, as though I were looking at a negative. Quick search of the web lead me to ATI's driver download site here (ATI Driver) and after a bit more digging I found the destructions on getting this running here Maverick Installation Guide. I ran through the procedure to remove old drivers, just to make sure, and then tried this install. Another forum pointed me back to the same guide, but because I was getting the "hardware not detected" issue suggested I manually edit my xorg.conf to force the fglrx driver. And now I get the (mostly) black screen...
First, please help me get back to where I can at least see what I'm typing, and if you have any pointers on getting this thing running, I'd appreciate them.
BTW I'm not sure if this'll make a difference, but I have both Gnome and KDE installed but I'm using Gnome as the default.