But the cost to download Parallels 7 ($80) and then there linked version of Window 7 , is bundled at $250!!!
The reason for that is that to run Windows on a virtual machine and clearly comply with the license you need to buy a 'full retail' copy of Windows, which is rather more expensive than the 'upgrade' (obvious) or 'OEM' (only supposed to be sold with new PCs or the bits thereof) versions that most people get.
If you find a cheaper way then I don't propose to lecture you, but you probably won't find such a solution in an advertised bundle.
As for the virtualization software, since VirtualBox is free its worth giving it a spin and seeing if it does what you want before you shell out for Parallels or VMWare - see if it does what you want. Parallels is slicker, and has some nice Windows/OS X integration features. Last time I tried, VB couldn't run the Aero style in Win 7 (some might regard that as an advantage) and parallels seemed to have better game support - but VB is developing all the time. I haven't looked at VMWare for ages.
As for people suggesting Boot Camp: if you want to do any 'heavy lifting' in Windows and need the full performance of your machine, that's the way to go, but it does mean that you have to restart every time you want to switch OSs - you'll also need something like Paragon's NTFS for OS X and/or HFS for Windows to exchange files between OS X and Windows, whereas VB/Parallels/VMWare include file sharing features.
Other pros/cons on an Air:
* Boot camp means deciding from the start how to divide your SDD space between OS X and Windows - which is fairly inefficient if you've only got a small-ish SSD. Parallels/VB make you specify a maximum disc size, but the 'virtual disc' file starts out small and grows as the disc fills up.
* Parallels/VB are memory hogs. Can't remember if any of the 2011 Airs only had 2GB, but if so... not so good.