I have a Macbook Air and a windows desktop that runs all my test browsers and a linux VM through virtual box. I use VNC to test everything on the browsers I have installed on the windows box and additionally run databases on the linux VM. Super smooth, I have never had any performance issues at all. Modern CPUs are more than fast enough to really do any type of development on as long as you have the ability to offload the occasional compute intensive tasks.

I also only have 4GB on air and yet I can't remember when switching to an application wasn't instantaneous even with 20+ applications open. I won't go back to running Vmware on the laptop, so much faster as a virtual desktop.