As far as files copying over if you upgrade, I would make sure that you have backups and backups of the backups coz I don't trust windows not to loose the files (and I've been playing with windows PCs since they weren't windows! lol)
As far as the specs for running windows 7, microsoft say:
If you want to run Windows 7 on your PC, here's what it takes:
1 gigahertz (GHz) or faster 32-bit (x86) or 64-bit (x64) processor
1 gigabyte (GB) RAM (32-bit) or 2 GB RAM (64-bit)
16 GB available hard disk space (32-bit) or 20 GB (64-bit)
DirectX 9 graphics device with WDDM 1.0 or higher driver
Have a look here:
http://windows.microsoft.com/en-us/windows7/products/system-requirementsHope this gives you more info, and not a headache.