What wiil you need to run arma2

look up the basic requirments and thats what you will need to get 20fps at minimum settings.
you really want to aim for medium or recomended requirments and for that you will likely need another 300bux minimum.

day z is now separately released as a standalone game with its own engine. I would not use arma 2 as a reference for the spec anymore.

The standalone (which is not out yet) is going to use a modded engine similar to take on helicopters which in turn is a modded version of ARMA 2's engine, so using ARMA 2 as a base to build for is actually the right idea.

$400.... you could build something to run it but it wouldn't be pretty on that budget you may be best off looking on Ebay for a 2nd hand rig.