Possible to crossfire 2 different branded 5770's?

Sorry I left out a bunch of details it would be on a gigabyte ga 58 UD3R board with 6GB triple channel ram and an i7-920

Should work just fine... They will default to the lowest speed if it is different.

You can crossfire them just fine. You want them to be the same clock speed though otherwise you will get a few performance oddities.

If the cards are different speeds, and you want them both to run as fast as the fast one does, you can flash the firmware of the slow card to make it as fast as the other. You'd want to test it in single card mode at that speed for a while first, obviously. If it gets too hot you'll get problems.

I lost some monitoring abilities when I mixed two cards of different make together. One card didnt have all the same stuff as the other.

Just make sure you have adequate power supply!!!

Many people look over this when adding a second card, some cards can suck up alot of juice.

PS calc:
http://extreme.outervision.com/psucalculatorlite.jsp