Tech Question: Can I connect any CrossFireX ready cards together?

I have a Radeon HD 4800 which works great, but I need another place to plug in a third monitor so Im thinking about getting another card for my birthday in three days. Does it have to be the same card or can I use anyone that is Crossfirex ready?
 
For example my 4800 and a  5770.
http://www.newegg.com/Product/Product.aspx?Item=N82E16814102858
 Thanks for any input!

No. Even if you somehow get it to work, the difference in speed would introduce an unbearable amount of so-called "micro stutter" - I wouldn't do it.
 
http://en.wikipedia.org/wiki/Micro_stuttering

nah your best bet is going to be to get the same exact card as you have now and use that and use the latest drivers.

In theory, yes, it will work. But practically you will get next to no increase in performance. If you still want that third monitor, but don't have a spare output, your best bet would be to install the card and not put them into Crossfire mode so that one card takes care of two of the displays while the second card takes care of the third. 
 
That or do what William said and get a second card of the same model.

yeah you could but using two different cards would cause a lot of stuttering, which is a NO-NO. 
 
if you wanna do crossfire, all of the cards should be the same.

no, u will need the same two cards. it would be cool if it worked like that.

Awesome! Thanks for the help. I bought my card a year ago and now its crazy hard to find, damnit.

sli or crossfire cards are very picky about having different memory sizes and speeds, you really need to have the exact same model in order for them to work.