Windows Experience Index = 7.9, then what?

The score will get higher. As it already did. The first "high" score was 6.

Microsoft updates the patterns and result values accordingly.

You can find more information here: http://en.wikipedia.org/wiki/Windows_System_Assessment_Tool

By the way - this is not a real benchmark - this is just a value that is based on some benchmarks and should be understood as a measure for "user experience".

You should use real benchmarks (like 3DMark http://www.futuremark.com/, Spec http://www.spec.org) If you'd like to know how fast your system is.