hi guys,,,,i have decided to buy my friend's Asus 4890 to crossfire it with my own Sapphire vapor-x 4890,,,,Is there any problem with compatibility? should the 2 cards be exactly the same? I have a 600W coolermaster silentpro which supports SLI and I am to some extent sure that I'll have no problem in crossfiring them,or will I? please help me