cadaveca, Thanks for the hints. Perhaps Intel would be a better route? Besides gaming, I also do a little CG. And you're right, 12% + 45% is hardly the performance of 1 card! The cards should be utilized more during games i would think.
anoobarak, My PSU is a 750W Thermaltake that supports SLI. I'm not sure if it could be that but it's worth a look.
Do applications like Rivatuner give the opportunity to increase performance (utilization) without overclocking the GPU? That would be cool. Cause even if I OC, as long as the cards are being only half-used I would see little to no performance gains.