I Have an XFX 8800 gts XXX edition GPU and it scored a 6.8 in the windows experience index. I recently just purchased a xfx 8800 gts XT (just the normal edition) and i put them in SLI. I ran my the WEI again and it only raised me to a 7.0 and ideas why i got such a small increase??
Some games aren't designed for CF or SLI configuration, so in some games u will not gain benefit from your SLI config.
Also, WEI is not reliable to test your system, use software like Furmark to test it.
I never touched Furmark with a 30 foot pole after I heard its kills HD 4000 series.
You might like the higher number but that doesn't mean it means anything, even proper benchmarking tools hardly reflect gaming performance. If you really want to see how much of an increase you got by adding the extra card then use Crysis' benchmarking tool and then take out a card.
If you really want a nice pretty windows score then you can set your own if you really want, a nice little search of mine found this...