Vista's Windows Experience Index was an easy way to see, in number form, which parts of your computer was responsible for your sluggish performance. Windows 7 does the same, but now it's better by two.
The new maximum component score is a 7.9. Microsoft has (and will continue to) raise the cap as new hardware components come out. If a current component score is a 5.6 under Vista, it will continue to be around a 5.6 (give or take) unless it was subject to various tweaks and feedback changes Microsoft took when adjusting the index, such as new disk tests.
...scores in the 6.0 to 6.9 range to support DX10 graphics and deliver good frames rates at typical screen resolutions (like 40-50 frames per second at 1280x1024). In the range of 7.0 to 7.9, we would expect higher frame rates at even higher screen resolutions.
But still, it's a bit of a clunky way to determine how well your graphics card will perform in newer games. Ballpark, sure, but nothing specific. For that, you should use more dedicated benchmarks like 3DMark.