Besides divulging the PC requirements for the third stand-alone S.T.A.L.K.E.R. game, Call of Pripyat, developer GSC Game World also launched the official site for the first-person shooter complete with a selection of new in-game images and some character bios.
Are they doable? Definitely. I'm no expert PC builder, but I'd be willing to bet you could very easily meet the recommended requirements for under $600 if you assembled the rig yourself. It's not until you bring visual-enhancing mods into the equation (assuming there will be some) when things start to get kind of expensive.