The current MBA i5/i7 are beats compared to the 2010 MBA. You will want 4GB memory on your air and at least 128GB of SSD. Give VMware/Parallels 1GB of memory and 2 CPU cores and you'll be set. Why? Because SSDs are extremely fast and will not be speed bottleneck like normal hard drives in your desktop Macs. Even 10,000rpm drives are slow compared to SSDs.
So you will not notice slowdown in your VMware/parallels on the MBA. Unless you need to run specialized programs that requires full power of the drivers (ie. Maya, 3D studiomax...etc.). Most normal applications will run fine on your virtual environment (IE, office, quickbooks...etc.). If you are running games, which I doubt you will be, look elsewhere for something with a dedicated graphics card.
Currently, I run win7 on my VMware fusion 4 on my 13" i7 MBA as its own space/virtual desktop most of the time. And I can just do four finger swipe and switch between Mac and PC environment. Just a heads up, running any virtual machine will decrease of your battery life very fast. So if you are planning to take it on the road running VM windows all the time, I suggest you getting a PC laptop.
And once you go SSD, it's hard, at least for me, to turn back and normal hard drive. Even my 27" iMac, I had to add the extra 256GB SSD as my boot/application drive and keep the 2TB hard drive as a secondary storage drive. Everything is much much snappier and fluent with SSD...
