Right now I am running Lotro in a window (just watching the chat screen to see if any interesting groups come up) while playing Left 4 Dead 2 while downloading Area 51 (Free Download, Ad Supported) while writing this blog post. CPU usage is 10% rising to about 60% when I actually start shooting Zombies.
The second screen is an old Dell monitor courtesy of my brother who recently upgraded his PC and was throwing this out. I hooked it up to my machine and I have fallen in love with dual monitors. Now that PCs are powerful enough to actually run more than one programme at a time I find it really improves productivity. When I am working I find I can review a document on one screen while typing my comments on the other. When gaming I can have a browser open on one screen while a game is being played on the other. One minor annoyance is that most games demand focus if you want to run them in full screen mode. Left 4 dead is somewhat unusual in that it has a border less windowed mode that behaves like full screen but allows you to change the focus to another application without minimising the game.