Parallels Desktop 9 for Mac lets you seamlessly run Windows and Mac applications side-by-sid

GPU teaming for Windows booting Mac Pro

Booting Apple's new Mac Pro with Windows may yield a special performance benefit not available with OS X. AppleInsider reports that when booted with Windows, the Mac Pro will support a GPU-teaming feature of AMD's graphics processors called CrossFire, which enables two identical graphics cards to work as one.

CrossFire can speed up graphics-intensive games and applications, according to AMD:

Applications such as Siemens NX, Autodesk Maya, Teamcenter and Ensight can see up to double-digit percentage performance gain when using AMD CrossFire Pro technology vs. a single card.

OS X applications won't be affected because Apple doesn't support CrossFire in OS X and AMD doesn't support OS X in the firmware of the graphics cards. Similarly, CrossFire would not work with Windows running in virtual machines with OS X as the host OS.