lemonys thank you for the info 12.3 works with both cards but with a performance hit, used to get about 310Mhash/s from 5850 now 250Mhash/s
& 510Mhash/s from 7950 now 480Mhash/s with the same amount of overclocking on both.Going to have a play & see if i can get the hash rate up a bit.

Thank for your help.  

UPDATE: hash rate is all over the place