yes you should be able to drag windows or drag the edge of one program window over to the other so the program-like adobe premeire or your editing program -will take up both windows or just have one monitor for playback.
Once you have two monitors, it seems impossible you ever had just one.
On the strain issue-I would not use a second card in a pci slot that would be used for editing. Too far down the bus=too slow-better if you could buy a single card (agp) with dual outs-usually one is a dvi so you will need a @$10 adapter dvi to vga unless your second monitor is dvi.