external monitor recommended resolution

if i have a white macbook 13.3", 2ghz intel dual core, with 4gb ram, nvidia geforce 9400m gt 256 mb, which would be the recommended (supported resolution) for an external monitor to use? I want to buy an external monitor to be able to work in paralel with some applications and i'd like to know a resolution that wouldn't generate require more video power than i have and would be laggy.
i will use the external monitor for: photoshop, dreamweaver, programming, working with parallels(basic apps - windows packet tracer, cisco courses), video...
thanks a lot.

anybody ... please?