Will the MacBook Air (2011) perform as well as my MacBook Pro (2008)?
The CPU will be adequate, but the MacBook Air is limited to 4 GB of RAM. This effectively gives you somewhere between 1 and 1.5 GB for your Windows 7 VM, which is not much. While the SSD will make for much faster IO than the HDD in your MBP, I expect you'll be doing a lot of pageouts.