What are the specs of your iMac? If they're good enough you will be able to play The Witcher (with Windows installed - there are some problems with non-native installations, in example using Wine).Oh, I forgot about one thing: I can play The Witcher just fine on my eight-year old PC (upgraded a bit some time after release of The Witcher). I can't play with high details but the game works fine. So I think you should be OK if your iMac is not too old 
Hopefully it will friend, so you dont have to spend a lot to buy a new PC.