The P5E3 Premium, like all x38/48 motherboards, has two 16x PCIe 2.0 slots.
There are two blue slots running at the full 16x, one additional black slot running at 4x or 1x.
Both a single 5970 and CrossFire 5970's will work just fine on your motherboard.
Make sure you use only the Blue PCIe slots for best performance.
TBH you could have found this answer very easialy using Google or by reading the SLI / CrossFire FAQ....
OMG yes you are right,all X38/X48 support CF @ dual 16x board that's why i have wrote in the FAQ too i didn't see the black PCI-E on this motherboard that's why i thought it doesn't support,it was strange for me too because i knew it should support both at dual 16x ,anyways thanks for clearing it up .
All X38/X48 motherboards support cf @ dual 16x so you can use 2 HD 5790's on it
thanks again outlw6669