VirtualBox or Parallels: Great Visualization
It depends on what kind of thing you want to do, but neither will be a good enough option.

If doing GPU-intensive work, your best and probably only option is BootCamp, and then Parallels.

VMWare Fusion would work, but it's slightly behind Parallels. VirtualBox is worst, but it's free.

As for whether you want Windows 7 or 8, it really depends, Windows 8 start up a lot faster, but consumes quite a lot more resource than Windows 7 until I turened off some useless services.

Here's a link you can check out if you like to see some benchmark result.

http://www.macobserver.com/tmo/article/parallels-desktop-8-vs-vmware-fusion-5-benchmark-showdown

And here's a even better one that compares all three virtualization software against Boot Camp.

http://www.macobserver.com/tmo/article/benchmarking-parallels-fusion-and-virtualbox-against-boot-camp