Without the 58XX series cards in the mix, however, they're pretty much it if you want a semi-reasonable $ / MH ratio.
Actually the 6950s don't do very well in terms of Mh/$ ratio. You said yours max out around 355Mh/s. Newegg lists the XFX 6950 1gb model, which is the card you mentioned, at $240, making the Mh/$ ratio just under 1.5 Mh/$. That being said, there are plenty of cards available now which easily clear the 2 Mh/$ mark.
The 5770/6770 cards (essentially the same GPU) do 220Mh/s and above, so if you can buy one for under $110 (which there are plenty of deals for $110 and less) you've broken the 2 Mh/$ mark, assuming you can push it past the hash rate I mentioned.
5830s can still be found, although they are becoming increasingly difficult to find. That being said I bought two a couple of weeks ago for $130 each and I have them doing 315 Mh/s for a ratio of about 2.4 Mh/$. This is about as high of a ratio as is currently possible with available cards.
5850s are all but gone from the marketplace, however I used to see these sell for $160 and it's possible to push them above 400Mh/s, making its ratio a solid 2.5 Mh/$. If you can find one, this is about the best you can do in terms of Mh/$.
2 Mh/$ seems to be the mark that everybody aims for, and there are plenty of cards that can do this. As for the 6950, the best bet for maxing out its Mh/$ ratio is hoping you can get a reference card whose shaders are able to be unlocked. This adds a significant performance boost into the 400Mh/s territory. Even so, the ratio would be under 2 Mh/$ but 400Mh/s is not something any card with a 2 Mh/$ ratio can hit. At a certain point, it becomes less important to get the highest ratio possible in favor of getting a high Mh/s rate from a single card.
