5770 x2 =

yep 2 x HD5770's roughly equals a HD5870.

They're a little better, but yes, the 5770x2 and the 5870 are virtually the same.

Theoretically they are equal,but you would never get 2x performance as cross fire/sli scaling are never 100%.So you can expect a performance in between 5850 and 5870.

^+1 What this guy said. Some titles it may match or slightly beat a 5870, in others it may drop to 5850 performance, depending on the drivers, and in completely unsupported titles, will only perform like a single 5770. Averaging it out across the board, 5770x2 does not equal 5870.

The only advatage is like what the others are saying,
its 2 instead of 1.
Just like the great debate we had around the store about if 1 stick of 2gig ram was better than 2 1gig sticks of ram, and the answer is always the same between 2 different things that are "equal to each other."
It really matters on what your using it for.
Theoritically, 2 is better because its like having "Two driveways"
but since the difference here is between 128bit and 256, Like JofaMang said, the pro and cons will about average out.
Hope this helps.

Its not like having two driveways at all, two driveways are useful as you can park more vehicles as a pose to only having one driveway, however 2 cards is more of a hindrance because they take up more space, will often use more power and produce more heat, vs a single card. Also it could be harder to sell two cards when you come to upgrade and crossfire doesn't always scale well.

= HD 5870

Two driveways take up more space as well.
You can fit to cars in a single driveway, usually.
Maybe I overlooked something in the specs?
It was supposed to be a bit rate comparison, but like I stated, 128x 2 is 256.
Im sorry I didnt specify better.
I wasnt considering re-sale value, wattage draw, or heat, merely numerical performace specs.

But omgitzfatal is correct, thgua

there will be factors like the ones he mentioned that might also play a role in what to decide in too.
the resale of a 5870 will always be higher than the 5770.