Vista sucks so skip it. Windows 7... always go for Ultimate, because any other version of Windows 7 is a hell no... to many restrictions. The only setback you'll have to do if you go for Windows 7 Ultimate is knowing what your motherboard model is and know its chipset, vga, and so-on so you'll not have a hard time finding the drivers. Though if your desktop sucks (in your opinion) or doesn't meet the general requirements. Don't go above XP. :3 Oh yeah, upgrading to Win 7 may give you trouble running some programs. Though it's bearable because it has its compatibility mode, not always reliable but does the job most of the time in program compatibility.

^sorry, anyway. checked the latest version of windows vista sp2 august 2010 and it's stable.