If you're planning on buying Windows 7 when it's released then why not just get Vista with a free upgrade to Windows 7? It costs less that way and you'll have access to all 4GB of RAM right off the bat. You can also do an upgrade to 64-bit Windows 7 from 64-bit Vista. You'll have to do a clean install if you run XP until you get Windows 7.
If you're avoiding Vista because you "heard" it sucks then I can tell you that I have never had any incompatibility or stability issues at all with it. It's bad rap came from the initial release when drivers caused instability or people were trying to run it on systems that weren't powerful enough to run it.