You didn't state your operating system.

If you're using Windows XP and care about decent frame rates, *definitely* get the same resolution monitor for your 2nd so you can run horizontal span mode. If they are different resolutions and you run dual-view, performance is horrible on the 2nd monitor. You *can* run the larger monitor at the lower resolution to have horizontal span, but you'll have some "hidden" desktop you can't access.

If you're using Vista, get a larger monitor and put the smaller windows on that.

I was running a 24" (1920x1200) and a 22" (1680x1050), and performance sucked on a 8800GTX, overclocked Q6600, and 4GB of memory in XP in dual-view mode. I managed to run 1920x1200 in horizontal span mode, but much of the area dedicated to the 22" display wasn't viewable. This meant I could move my mouse off that screen and it'd disappear into what the computer thinks is viewable area. Tough to explain, but easy to see.