Hello Luis10689!
I currently run Boot camp On a 27 inch Imac, With OSX as my other OS that, The easiest way, IMO, is just simply boot camp, its included and the boot camp assistant should walk you through everything you need to do, But, If you wish to use Vmware Or parrels,,(I be warned, I heard it doesn't use ALL your system specs ( EDIT: Thanks to jay for straighting that out for me: Jay "No, it does use all your system specs, but virtualisation programs have the overhead of emulating real hardware, which makes them slower (unless it's paravirtualised, but that's more enterprise-grade virtualisation..)" , so any high CPU intense/GPU intense Applications (such as Games, Like gta.....) might lag, Again, I found the easiest way to just simply install windows, Im using windows 7 currently through boot camp, and really, haven't had a problem (mac related problems, good ol win 7 has been, actually, decent to me) Since, Also, I suggest WIN 7 over Vista, And yes, as long as you know what your doing it should be rather simple to install modded cars onto Gta IV (there are alot of tuts, ill link you to a couple upon request). If you have ANY questions, feel free to either pm me, or reply here, I will be glad to help you with any mac/bootcamp/windows related problems,
Regards,
Justin, AKA JAM.