PhysX is polluted by a bunch of marketing BS from Nvidia.
If you are looking to dump a ton of time into a game that uses PhysX well, then you should absolutely have an Nvidia card. However, don't buy the marketing garbage - few games use PhysX and only a couple actually use it for anything meaningful.
The crossfire problems are what I expected and why I recommend going with a single GPU 660ti. (I'll be going to a single GPU once one exists that does 7970 CF performance, for the reasons you've stated)
I'd say grab this one for $300:http://www.newegg.com/Product/Product.aspx?Item=N82E16814127696