One thing that caught me out when I swapped from ATi to nVidia is that using a DVI lead I only get a signal out of one sockets on the back (the lowest one, closest to the motherboard), the other stays off until I get into windows. This even happens with only 1 monitor connected so at first I thought I had a duff card.
I think that's normal. My 8800GTX feeds two monitors. The second monitor starts getting signal only after Windows has booted. I'm guessing it's because the drivers have to load first and notice I'm running DualView.