I love my iMac, but it is NOT blazingly fast. My friend who works at the store said his newer one and the brand new ones are not much faster. So for blazing speed you might want to consider one of the towers, or pony up for the top dog quad-core 27" (I saw refurbs are now $400 less which some take as an indication of new models to come). Also 27" seems better for video editing.
My old unit has 2GB or RAM and it can start to run out with a bunch of Safari windows and Excel and iTunes open, so from another thread I'd say you should be at 8GB.
Unlike PCs, whose constantly greedier software obsoletes the machines quickly, Apple software is much more compact. One result is a Mac is "Still usable" for far longer than a PC-six years plus in my case. So don't be penny wise and pound foolish-spend the money for a really good machine.