A 280x is the same as a 7970. This means a 280x is GCN 1.0 so it only supports MANTLE and FreeSync. A 280 is a 7950 so it is also GCN 1.0 .
A 260x is new, so it is GCN 1.1 which means it has improved MANTLE, True Audio, and bridgeless crossfire, and everything GCN 1.0 had.
A 285 is newest and has GCN 1.2 which has all the features of previews revisions, and has a 4k media decoder, and is more efficient.
You cannot crossfire a 260x and a 280x. You can crossfire a 280x with a 280, 7950, or 7970. Some vbios tweaks may be needed for the 79xx cards to work...
A 280x is more powerful than a 285 or 260x, but is running on older architecture. If these features of GCN 1.1/1.2 are important to you, get a 285 as it isn't that less powerful than a 280x, but runs cooler, costs less, and supports new features.
If you want something more powerful than a 280x that also has all the bells and whistles, then get a 290 or 290x.