Windows 7 is available and has been well-received (especially compared to Vista). If you are running Vista, you should upgrade.
The tricky part for consumers is that in typical Microsoft fashion, they have released a number of different versions. Which version is right for you?
Fortunately CNET has released a chart of functionality that compares the four different versions.
Check out the full chart; it’s extensive yet digestible.
Only you can determine what version will meet your needs, but my basic suggestion is that “Starter” is just too lean, and “Home Premium” should suffice for most day-to-day users with an average PC. If you are running a beefier powerful computer (two physical CPUs), you need “Professional” or “Ultimate.” If you are a power-user… figure it out for yourself.