I'll be using parallels 6....
So its ok to run xp 64 bit on SSD?
Good idea. I run Windows 7 in VMware Fusion's Unity mode. That way, I can keep Windows apps open simultaneously with OS X app. I use Quicken for Windows several times a day and leave it open on my desktop with Chrome, Mail, iCal, and Address Book. I also use WordPerfect 12, another Windows app, inside OS X. I believe Parallels can allow you to run Windows apps in the same way Fusion's Unity mode does.
The conventional wisdom is that running the 64 bit version of Windows in a virtual machine is a waste of resources. I agree. As a practical matter, you would never dedicate more than 2GB of your MBA's RAM to your windows VM. To start getting any benefit at all from the 64 bit version of Windows, it needs at least 4GB of RAM, which, for obvious reasons, isn't happening in a VM on an MBA.