Running anything in a virtual machine means you are typically going to take a hit. VMWare has been doing a lot of work to get direct access to video cards and processors for performance gains, but this breaks the sandbox of a virtual image; so it's a difficult problem. They are working on it.
That said, the best you can hope for, for games that are not supported natively on Macs, is to run a virtual image (with VMWare as an example) and play the game in it like you would with any native PC. It may work great, it may not work on the lowest settings, you really just have to try it and find out.
Edit: Alternatively you can dual boot your machine, and boot into Windows whenever you want to play the game. This would eliminate the need for a VM.