Dual X8 is the limit for P55s and it shouldent be much of a performance hit.
The CPU is fine also. 3.8Ghz i7 shouldn't bottleneck anything.
Did you reinstall drivers and clean out the ones for your old card? Or is this a clean build?
Also is SLI enabled in the Nvidia Control Panel?
This is a clean build, at first I had one gtx470 but then I got another one and installed it with the drivers a week later, should I scrape everything and start from scratch -drivers-wise? In that case how can I clean them out as you said?