I checked my WEI just now and I think my graphics rating went up too. I believe it was 7.2 for my 4890 when I had Windows 7 RC installed but now its 7.6. I installed the full version of Windows 7 this morning but didn't notice the change until I saw this thread
WEI scores were adjusted between 7 and Vista to accommodate the new scale. The maximum score under Vista was 5.9, whereas under Windows 7, the max is (currently) 7.9. Naturally, existing scores had to be adjusted to reflect proper values to avoid getting older high end cards with lower scores than newer mid-range cards, and whatever other WEI score anomalies one could think of.