I'm running Win7 here with Catalyst 11.5's and SDK 2.4. I had a bear of a time getting things working right on Win XP so I went with 7. I've also found that the AMD drivers can get a bit wonky at times. If you try to do uninstalls/installs, sometimes crap just gets gunked up and I had to format/rebuild.
Make sure you have more than enough power for both cards on your PSU or you could see all sorts of problems, especially if your PSU has more than 1 12 volt rail. You could be overloading a rail and if one of the cards doesn't get enough power it might not start or it might work half arsed...
We'll I guess I'll see what happens tomorrow when I piece it all together. Hopefully it all works properly without much issue.