Crossfire with 4870 x 1gb and 1 x 512 problem

[philip j fry]

Ok have tried both these cards in xfire and does'nt seem to work or just not getting any proformance gain , have tried everything i can think of i.e different drivers ( tried 9.1, 8.12 and 8.12 hotfix ), have used a single xfire bridge and also both bridges , tried in opposite pci-e slots just does'nt seem to make a difference. In ccc when i have ticked the xfire enable box a message comes up below saying "these 2 cards r not a perfect match but will still be compatible in xfire" gpu-z see's them as running in xfire as well but just no difference in games or in bench marks.When i run any application with xfire enabled only 1 card seems to be working . Tbh i was thinking that these cards just were'nt compatible in xfire but ccc enables them so not to sure ,the only other expaination would maybe be my psu its a thermatake 750 watt has 3 main 6-pin x pci-e leads and was using 1 x 4 pin-6pin lead which comes with the card , there was nothing else plugged in on the 4 pin lead was straight from psu to 6-pin adapter.
Hope some1 can shed some light on this for me cheers
Also running on vista x64