ok, i already own XFX 5870 card, and im planning on adding another one. Does it matter about branding of the card?
for example, xfx and asus on CF? or xfx and sapphire... or going with same brand xfx and xfx is better solution?
I don't think it matters. But, personally I'd stick with the same brand because its consistent. Especially if you want to sell them later, I personally wouldn't buy 2 different branded cards for my system. Patience
Here is the golden rule: In crossfire, all the cards run at the speed of the slower card.
So YES, there is a disadvantage to pair 2 cards not the same speed. So technically, an ASUS 5870 paired with an XFX 5870 would "downgrade" the ASUS model to run at the same speed than the XFX one. The ASUS 5870 is clocked higher than XFX.
However, NO you won't see any real impact in this particular case. The reason? Except the ASUS 5870 I don't know any 5870 that aren't a sticker over the reference design. What I mean is that all the companies seem to have just put their sticker on the AMD/ATI card, they have all the same clock speed, the same fan, the same everyting. Based on that, current 5870 are all the same, so there won't be any impact using another brand. Even the ASUS 5870 is only a slightly overclock "same thing"...
Personnaly, if I had an XFX, I probably want another XFX just for the sake of having to deal with a single company if anything happens. The fact that XFX offers a lifetime warranty would encourage me further to get a second XFX.
If you got the money and can't wait, buy another brand, it doesn't really matter in THAT particular case.
Well, just to calrify fully, I have two 5870's, one is HIS and one is Sapphire, they are completely identical, the cooler and stock speeds are exactly the same. The only difference is the stupid sticker they choose to put on the card, and of course their return policies etc.
Basically it shouldn't be a problem at all. Even if one has an aftermarket cooler and higher stock clocks it will still RUN just fine with the other card, but it will be very very slightly hampered, unless you OC the other card to match specs, which should be easy to do and quite feasable. Hope I helped.