Depends on what applications you intend to run but I assume InDesign, Photoshop etc. Virtually any new mac will do. Your cheapest bet is a Mac Mini. the latest ones come with 4Gb of ram and it means you'll be able to keep the monitor you use on your PC (although you may need a cheap adaptor). Also it will come with the latest Mac OSX which means that through Boot Camp (included) you'll be able to use your PC programmes on it as well.
