From a cost to performance standpoint. I'd say another 285 would save you money, and you'd get close to the performance of a 5970, although still a little slower. You maintain physX at the cost of DX11.
Mind you, 2x5970 will need a new PSU. As far as heat, you can gauruntee that running 2x300w cards will make more heat than what you currently experience. Is you case ventilated enough to deal with it? I don't know, but you may need to do some extra cooling to make it work should you invest in that route.
Othewise, a single 5970 would probably be a true plug-n-play option.