You need a full copy of Windows 7... not an upgrade. Two ways to buy it. Retail package or OEM. I usually buy OEM, but that doesn't get any support from Microsoft as the retail copy does.
Next, you have to choose between 32-bit and 64-bit Windows 7. Apple doesn't allow us to run true 64-bit OS X on the MBA, but I have been running Windows 7 64-bit flawlessly. At the same time, it's probably not necessary and adds to complications of drivers and app compatibility.
If you have limited tech experience, I would recommend a retail 32-bit Windows 7 full disk. If you have tech experience and you're willing to work to get everything running, buy the OEM Windows 7 full disk in 64-bit. One thing is Apple doesn't have any drivers for Windows 7 yet, although they promised them by end of calendar year. As Apple usually over-promises and under-delivers, doesn't seem it will have the drivers on time. So, for the Windows 7 64-bit, you have to go in and manually install all of the 64-bit drivers from the SL disk... which is actually Vista support not Win 7.
Finally, you can use Boot Camp for free. I highly recommend Boot Camp not because it's free but because it doesn't require splitting the limited resources of the MBA between two OSes. The MBA only has 2 GB of RAM, so designating 1 GB to each OS seems insufficient. Using Boot Camp allows you to run one OS at any time and that means you have plenty of resources for whichever OS you're using.
You could use Parallels or Fusion, but either of those is going to have you run Windows and OS X at the same time. I just don't believe it's a smart move with the MBA. If you had an MBP with 8 GB of RAM, it would make far more sense.
Would be a fun Christmas break venture... my experience has been a rock-solid performing Windows 7 64-bit, but the OS X side has been a disaster ever since I upgraded to 10.6.2 and latest Safari. If your experience goes like mine, you will probably enjoy Windows 7 on your MBA far more than OS X... I know it sounds backwards... it's confusing to me too! I always had loved OS X, and Leopard was amazing... I cannot say anything good about OS X Snow Leopard other than the Snow Leopard desktop background image is cool and it saves a little drive space... although that's misleading too.
Good luck!