You probably won't notice any performance hits with a 64 MB card. When you game, the secondary display will just go black, since there aren't any games that can use 2 monitors (that I'm aware of).
There are a handful of programs that fully utilize dual displays. Softimage and Combustion are a couple that come to mind. In fact, Combustion has different preferences for multi-monitor setups (up to 4 I believe). Final Cut Pro on my Mac has multimonitor settings as well.