HMMMmmmmm I wish to do exactly the same sort of thing - no doubt there are some wrinkles in my setup though.....
First of I am presently using a 9600XT (also have another machien with 9700pro) Second I make use of the PowerToys virtual desktop, just to complicate things, lastly I am not presently using anything other than native XP support for extended desktop with these cards (and their drivers).
I am on Catalyst 4.10 I think, maybe earlier on this machine though, would need to check but think irrelevant.
Do I need extra software to spread the wallpaper? You see I would really rather like to have Monica Belluci or someone like that reclinign on my extended desktop wallpaper (sad I know! I am sure I will graduate to Manhattan skylines and such eventually!)
So guys - do tell - I am kinda new to having the two monitors up - just got the second one as a birthday (& Christmas) present! So I would like to make the most of it and doubling up the same wallpaper just looks a bit sad - off to experiment now and see what I can do though!