I actually upgraded to Windows 7 a week ago. I was running XP. BIG improvement over XP. Only problem was that I needed to upgrade my video card. You need at least .5gb video ram to run Aero. So, i got a new one and then realized i needed a bigger power supply, so I replaced that too. All told, it cost me $350 to upgrade. I could have bought a new machine for that $$ (or 2 visits to my favorite AMP !)
Anyway, like i said, big improvement. Much faster and more reliable.