You'll probably be happier with the minimum frame rates and single gpu simplicity. 2x5770 looks to be about the same as a 5870, and the 6870 is just a bit slower. So if you can seel your 5770, its probably a good idea.
ok... i just took a look at your mobo
is it the GA-MA785GMT-UD2H??
because its a micro atx board with only 1 pciex16 slot
you need two of those to cf unless your planning to buy another mobo...
if your just asking for the fun of it...
i really think its about personal taste
take me for example... i would love to see a dual gpu setup inside my rig!
plus 2x hd 5770 is probably enough gpu power for antyhing you throw at it...
If you already have one 5770 and a capable crossfire motherboard then yeah I say just get another 5770 to run in crossfire. Of course if you're going to use it with that GA-MA785GMT-UD2H then you would need a new board with two PCI-E slots (that both run in 16x or 8x) to do crossfire. If you don't want to use a different motherboard then you are better off with a single 6870.