Performance issue solved in my case!
many guys over at my french thread report "cpu overclocking pb" once hd5770 is mounted! freeze/crash...They were compelled to clock down the cpu in order to run correctly the hd5770.
Changing the psu pc p&c from a 500w to a 610w did not solved the pb for me!
Partly solved by pluging-in fdd power connectors to feed further the pcie slot (a particularity on the dfi LT x48 t2rs) but totally resolved by down-clocking the fsb from 455 to 450!