Most cost effective way would be a good Windows 7 machine running the same Adobe software.. if you need other mac-only software then do a hackintosh – better specced machine for half the money.
This is true, particularly if you are working purely in isolation, once in the software the platform is pretty much irrelevant.
I have had some odd results with a couple of typefaces cross platform in flash, leading in particular seemed to go completely out of whack on the PC whilst is was fine on the mac. Same files, same fonts, completely different appearance. Only happened with a couple of typefaces but I never did get to the bottom of what was causing it.
It can create a bit of a hullabaloo turning up at a exclusively mac agency with a PC though which may be a hassle you don’t need. So long as you know how to integrate your PC to their network then it shouldn’t be a real problem, but don’t expect to receive much help from anybody else in the studio if you do run into a platform related issue.