The best performance comes from two monitors the same resolution in horizontal span under windows XP.

If you are going to have different sized monitors requires, this requires dual view. XP is bugged and has slowdowns on second monitors, Vista is not bugged and has good performance.

Frankly after knowing you are going quad core. I would then figure out your monitor setup/s. Figure out the exact size/resolutions you are going to use. After deciding that you can then decide on what you need to make the setup work best.....

4 toons on a 4:3 19" monitor will be pretty small will you be ok with that?