OK, now that is weird. I got single HD4850 1GB and in the windows experience index i get 7.3 on those two sections!
. You should be getting 15+ score with 2x5870 LOL ( i have win 7 ultimate x64 )
Anyway it must be a bug or something. And as the other guys pointed don't trust Windows Experience Index.
Your cards are great and the performance (probably) is and will be improved on the 5xxx series with the new drivers.