I'll let the other techies answer the comp specific questions, but imo you should definitly go for two screens.

Currently I play on two pretty standard 19"monitors, left monitor is for my main - maximized (set up with keyclone), with top graphic settings. The right monitor has my other four wow clients, all equal in size with low graphic settings. My reasons for having it this way is so I can still experience the nice effects and areas through my main, but can also easily see whats going on with the other four with a quick glance, and if it's needed jump to their screens without fuss.

Hope this helps,

X