Is the 5670 a good OpenCL platform? Would it be a decent card to upgrade to if you are unhappy with the poor OpenCL support with the 4850?
I would go with a 5770 ... over the 5750 or the 5670.
It has double the stream processors of a 5670 and more than 10% higher clock for less than twice the price. Its has more than 10% more stream procs than the 5750 and also a 20% higher clock for about %15 increase in price. It is like a 5870 with half the processors (same clock, just no double precision support, which I guess most don't need) and 2 5770s are easily cheaper than a 5870. For GFlops/$, I think its the sweet spot.