Vista... whodathunkit
Well, installing Vista really worked wonders.
Not only did it list both adapters for the 4870x2 as enabled (well, a primary and an enabled), it also started using my ATI 2600HD card!
You see, I have a Mac Pro and was using the 4870x2 through bootcamp. I have one monitor plugged into the 2600HD (which came with my mac), and one into the 4870x2.
In XP, once I had the 4870x2 going (with only one core working), my 2600 monitor would be "on" with a grey screen, but unusable. In Vista, I can do extended desktop. It's really quite amazing, it's the first thing I've seen windows do that the Mac won't - use two different video cards for extended desktop.
In any case, now I am getting performance from both cores for my games under windows and am very happy.
I'll ask you all about the AI Overdrive soon, because I don't think I am doing it right or something, but that's for another day.
Thanks to all for the help!!
Brad
EDIT:
What I am not happy about is ATI's support. I had opened a ticket about this problem and they informed me that it was "normal behavior" for the second adapter to be listed as disabled if a monitor was not plugged into it. THIS IS BLANTANTLY FALSE. It was not using the second GPU. Now, in Vista, -without a second monitor plugged in-, the second adapter is listed as "enabled".
I feel bad for anyone that doesn't know much about computers, because if they had received this kind of support i'm sure they would have assumed it to be working since there was no real "error" reporting anywhere.
Shame ATI, shame.