You're probably not going to like my suggestion.....

It's always best when running any kind of CAD program or graphic intensive game to run it in native Windows not a virtual machine. Any time you run a Windows program in a virtual machine it has to rely on drivers supplied by the virtual software; in this case Parallels. Although Parallels and Fusion both have improved virtualization methods especially with graphics, it's still "not there" yet.

I would do two things:

1. Setup a dual boot with Windows 7 and OS X. Use the Boot Camp assistant.

2. Install Windows 7 x 64 bit not 32 bit. Windows 7 x 32 bit can only utilize approximately 3.6 GB of memory whereas the 64 bit version will take advantage of all of your 8 GB.