Based on my perception Unity compared to Single window it seems to me that Unity is more resource intensive, at least CPU wise, and I wouldn't be surprised if unity is also more memory hungry since there are extract window objects to keep track of. I would expect that a Single Window VM will demand the least resources from you MacBook Air.
Also, uncheck 3D Graphics Acceleration, only select 1 virtual CPU, and start with RAM around 512 MB for the VM. Others are reporting they run with much less RAM allocated to the VM, but my experience with Windows in general has me puzzled why they aren't running into situations where the Windows VM starts to swap itself to death. I can see 384 Meg, maybe 256 Meg, but not below that for Windows XP SP3.
