I'm not sure if they're older, but I have several 5750s mining with smelter, and they work very well.
What kind of hashrates are you getting? Solving any blocks?
My 5750s get about 1.38MH/s each; my 5850s get about 2.5MH/s each. (I overclock them.)
All my cards together amount to about 18MH/s, and I find plenty of blocks--about what I should according to the estimates on pools. Of course, there is a huge amount of random variance in solo mining. Some days I find twice as many as I should, other days I find just one.
It is still the most profitable use of these cards, but that may change soon, as the price drops and the difficulty rises.