Re: MacBook Air 11"
A consideration is to go with the 13" as you'll have a bit more screen real estate to run your Windows VM and any OS X apps side by side. If you have a separate external monitor or just want to exclusively run Windows in full screen mode (Bootcamp or Parallels), then the 11" is probably fine.
If you can, I'd go for 8GB RAM so that you can allocate ~ 4GB to the VM. Tools, apps, and related work stuff is probably going to be happier and snappier. Parallels has probably optimized virtual RAM fairly well, but if you can avoid having to swap at the VM Guest or host OS level, your experience will likely be more enjoyable. As Rayz noted above, if you go with Bootcamp option then the physical RAM partitioning isn't a factor and 4GB RAM may be sufficient for running Windows 8.1 in Bootcamp.
Another consideration is the MacBook Air 11's local storage capacity. If your Windows 8.1 VM image size is ~ 100GB for example, then with a 250GB SSD in the Air you'll probably have sufficient disk space for OS X and the VM. Not sure what the $799 Best Buy configuration has.
Of course the considerations above mean possibly more expense or larger form factor than desired. In that case, the 11" with Bootcamp is probably the best path. Disclaimer, I've never used Bootcamp, just Parallels.