I have used my second screen to display system info and task manager for years while playing FPS's online and it makes basically no difference.
Saying that, the things I display on screen B are relatively static, with very little graphical change and hence no real extra load on the GPU. If you are playing online and watching a film on your 2nd screen then you could expect choppy performance, but not 'lag' in the real sense as it would have no increased load on your internet connection.
Snowdog is right as well, you should have a look through the processes (you can do that on screen 2 while playing) and identify any that are hogging cpu time or exessive ram. Vista, and now 7 are both extremely resource hungry and will take any opportunity to jump in on your ram and cpu time.
As well as the other nVidia options, for me (I have mismatched monitors) it was a matter of finding a compromise setting between them. I decreased the refresh rate of A to 80Mhz as that was the highest B could do at my target resolution and still look good. I can play X3TC without AA and watch a film at the same time, with AAx4 it does start to get choppy in combat. If no film is on, just task manager, then AA can go up to 8x no problems.
If thinking about the load from the GPU to two desktops it is best to either balance them (ie both under same load) or bias them (ie one for game, one for task manager). Splitting them (ie two different GPU intensive tasks) can create performance issues.
|/ or \| or \/ from the GPU is ok but || from the GPU is bad! Thats how I remember.
Nice PC btw!
