CrossfireX with 2 Different 7970s?

If the base clock speeds are the same, is there any difference putting an HIS IceQ X2 7970 and an XFX Double D fan 7970 together in crossfire? I bought the IceQ X2 one, but wondered about putting two different ones together. Is that ok?

As long as they're the same clock speeds, you won't have any problems at all.

I don;t think the clock really matters, does it? source?

Last i heard they had to be the same core. EG. a 7950 (tahiti) + a 7970 (tahiti) will work. 

From what I understand it isnt major to have cores running a different speeds but if their both at the same speeds, it will improve frame timing.

Just crossfire the same brand/SKU. It's aesthetically more appealing. But that's just me because I'm sort of OCD.. lol

I crossfire cards with diffrent stock clock speeds.

just find a clock speed that is stable on both cards. go as high as you can. You should not run into problems, only when you specify a clockspeed that one card cant handle.

well you can CF a His iceQ X2 with a XFX DD, the only big con is, that the HiS is an unlocked card, and the XFX is a locked card. so you will probably run into a down clock on HiS, because your setup will be running on the speeds of the slowest card.

i would personaly recommend the CF the HiS 7970, with an unlocked card like Asus 7970 V2 or Msi 7970 Twinfrozer. or Sapphire Radeon 7970 OC with  boost edition. Then you can overclock both cards.