Two monitors or one monitor and one VGA dummy. See link below on how to make them. http://soerennielsen.dk/mod/VGAdummy/index_en.php
Or if you have the latest drivers 185.85 you can add a force command in the folding link. "-gpu x -forcegpu nvidia_g80". x = you graphic card, for instance -gpu 0 1 2 etc. By using this method you dont have to use dummy plug and nvidia_g80 works on all nvidia cards. This -forcegpu command even works with ati x2 ones. Just use "-gpu 1 -forcegpu ati_r600" or something similar. More is on the foldingforum.org. EDIT: Ah see the OP is on Ati card but this will work for your card ass well. Its built in Cuda 2.1 application now.
The ATi variant will use CAL instead of CUDA. I have Folding@Home running on my Radeon HD 3650 (desktop) and my AMD Athlon X2 3600+ (media centre). deltatux
You don't necessarily need a vga dummy to accomplish this. Just a monitor that has both dvi and vga inputs, and is able to switch sources. It's been some times since I've folded so I've forgotten what to put in the command line for the f@h gpu shortcuts. All I remember at the moment is that you need two separate shortcuts.