I've got 10gb of PC6400 in my 2008 2.8 8-core MP. I've also got an extra 10gb of PC5300 ram from an original MP. I've read somewhere that the PC5300 will work in the 2008 MP. All the ram will run at 667Mhz if I add the PC5300 to the 2008 MP. I guess the bus would also drop to 1333Mhz from 1600Mhz. Will this produce errors or will it only reduce the bus and ram speed? I'm running Adobe production premiere so running 20gb of ram would be much more beneficial than running 10gb of ram even if it meant a slower bus.