Do you already own an ATI 5670? If you don't, it's better to just buy a single ATI 5770. As two 5670's in Crossfire perform about the same as a single ATI 5770 does. In some cases the 5770 is actually a bit faster than two 5670's.
ATI's Crossfire does allow you to match up two cards that aren't the same model. However the 5670 and 5770 perform so dramatically different that I don't think it would work all that well. Considering a single ATI 5770 performs equal to or greater than two 5670's. You're better off installing a single 5770 and selling your 5670.
You can crossfire the 5670 assuming you have a crossfire board. You do not need a crossfire bridge for that card, but it would probably help . Anyway, if you already have a 5670 then adding another one would be a great way to increase performance. If you want to buy two 5670s now you are better off getting a single 5770 or GTX 460.
I don't know what to make of that chart, it isn't exactly all accurate.
As in your results were different? Or as in you think Guru3D made up numbers and lied?
One of those reasons is invalid if you ask me, and the other is a crock. So you pick. Guru3D is a pretty reputable site, so I hope you have some evidence to back up your claim that their chart is not accurate.
Yes the results will vary on every review site online. However, they should generally paint the same picture. And that is that the ATI 5770 is a better option than two 5670's. Just the way it is.