yo dawg, i heard you like emulating

Our best bet is the Wine project:
http://www.winehq.org

It's not an emulation,, so performance can be quite good even for more modern games. Some of the "Mac versions" or PC games sold by GoG are just Windows games wrapped in Wine.

Well there is the idea of porting Wine to WIndows. That would basically cater to the same needs.

which is great, isn't it?
just tried some gog games (specially primordia and other ports such as system shock 2) on a friends mac and damn, it's a charm

I think Primordia is a native port, not Wine involved. I've been using Wine to play Windows games on OS X for quite a while, but it's always great when GoG does the work for me and I can just download and play the game. It's really unbelievable when you think about it, we have all these incredible games that we can play on systems they were never designed to run on.