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.
Hope that helps.
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