x48/P45 for a 65nm quad core will oc about the same. P45 will oc dual core 45nm cpus thru the roof, sorta speak. X48 is more suited to ocing 45nm quad core better then P45. though it has been a while i might have that backwards.
now if you want to just compare ocing aspects of 790i VS P45/X48, always take a intel chipset based board. dont get me wrong NV based boards for intel have some nice features besides SLI. unlocked ram clocking is one major one i can think of and honastly about the only one. NV based board just need more nb voltage to oc vs intel. on top of that NV based NB's run hotter as well.
Honastly i would keep what you have and get just a single high end gpu. I wouldnt bother with SLI or CF, though i do happen to run CG atm. it is just to hold me off till prices drop a bit on the new ati 5000's. as there is no sense into buying 790i just for sli with your current quad core. as then with 790i you need DDR3 ram for it, at that point with a new mobo. You might as well just go AM3 or LGA1156 setup.
