I would go with 280x, its nearly 2x faster anyways, plus I've heard when you "SLI" it becomes half the size of original (i.e only 1gb useable on 2gb card because it needs mirrior image of 2nd card, tho its rarely a prob with most games), where 280x is 3gb, some newer games need that for ultra textures.
It doesn't become 1GB in SLI. What it does it's that the memory on each card have the same data. So if you have two 2GB cards in SLI, available memory for games will be 2GB - it doesn't stack. On the other hand, each GPU core is working on its own frame, so the core computing ability adds up.
Doing SLI/Xfire with low vram cards will result in the high core potential held back by low amount of vram.
Stacking 2 gtx660 will result in a core performance around r9 280x, but with 2GB of vram which is fine for most game in 1080p.
I would wait and see if r9 285 brings something to the table as it seems to be in the performance range.
I got a good deal on a 660(non ti) summer of last year for $100 used and its been great but its time for a upgrade. I seen a used 660 for $90 shipped I was thinking of picking up but then I'm seeing used 280x's for around $140-$160 shipped(minning cards of course). I know its always best to go single card solution if building from scratch but in this case I'm not. Tried searching but it seems most are asking should they go sli from scratch(not owning one) or 280x.
If that was the case I'd go 280x but I already have a PSU,mobo,ect that can handle another 660.
So you need to pay $90 to have 2x used gtx660 in SLI?
Or you can sell your 660 for $90 and add $50-$70 to get used r9 280X?