Grab this
driver for OS X that'll read and
write to standard, NTFS-formatted hard drives. Then move all your data, or use Time Machine, get a MBA, and restore the image onto the MBA.
edit: As for a decision, I used to be PC-only, and gamed on it all the time, then I went to school. It was a Mac or a gaming notebook, like an ASUS or Sager. Went with the 13-inch mid-2010 MBP. The 320M can handle a lot of things I throw at it, but it's not that great. The Air has the same card, and I can pull off 20FPS at the lowest setting the Crysis 2 demo allows, but the Air also has a < 2GHz Core 2 Duo. Unless you want to be playing everything at low- I'd suggest against an Air. If you really love OS X, go grab a new 15-inch. The video cards in there are blazing fast, and so are the processors.
However, what Butterfly suggests is the ideal setup, IMO.
edit 2: Or, you can sell your iMac and get a 15-inch or 17-inch MBP with a 6750M video card. This'll also come with the newfangled Thunderbolt port, which I doubt you'll ever use, but who knows?