Asus and Sapphire in Crossfire?

FAQ is your friend:
http://www.tomshardware.com/forum/ [...] sfire-faqs

What about brands ?
Well the brand doesnt matter , again for example , you can use a XFX card with a EVGA card , or a SAPPHIRE card with a DIAMOND card , just make sure they have the same memory and same clocks.

What will happend if 2 cards have the same amount of memory but one has higher clocks than the other?
One of the best ways to learn is to make examples.
You have a 8800GT(Normal edition,no OverClocks) and you want to add another 8800GT(Which is OverClocked),then:

1_The OverClocked one will lower its clocks to reach the Non-OverClocked card.

2_You can OverClock your card(The one that has Stock frequencies) and reach the OverClocked card.
Remember, you cant ALWAYS OverClock the normal card to reach the other card,because it may not OverClock that much and may become unstable.

Thanks mactronix yeah i didnt see he was talking about X1900,well i have that in the FAQ too :
Warning: AMD/ATI X1800/X1900/X1950 cards need a CrossFire-Edition of them to run in CrossFire mode.But for HD Family its different, they don't need CrossFire-Edition, they just cant work with each other for example a
HD 2600 works with another HD 2600 just fine.
Also please read the note about CrossFireX at the first parts of this guide.

No problem mate
and thanks for u for mentioning that it was a X1900 card