I'm playing with 4100 DPI. I think it's just a matter of preference, although a really slow mouse speed might be penalizing.
4100 DPI lol
DPI is about the speed , depend your screen size and your windows settings should be 6/11 (for no acceleration)
if you use too low dpi you will lose time but if your dpi is too high you will not be accurate.
I personnaly use 2000 dpi on 15,4 screen with 6/11 windows settings (was optimized for SC2), but can be play at 3000 i guess for COH 2( 4100 is much too high using my settings)