Which windows version runs best in Parallels?

I am currently running 10.6.8 on a 2.53 Ghz MBP with 4mb ram, and Parallels Build 6.0.12094 with Windows XP.

It works fine, but is a memory PIG and I pretty much have to reboot after I shut down Windows.

I have a new Air on order, and am wondering if I will be better or worse off with a different version of Windows.

Also, is there any way around having to email myself a file I need to transfer from XP to MAC?

Thanks in advance for your help.