Well, you question is kinda skewed given that the $300+ cards are overpriced right now. The $230 6950 is the end of the good price/performance cards.
You should prioritize a single card over CF/SLI because CF/SLI can have scaling issues, low minimum framerates (which don't show up in average framerate benchmarks), straight compatibility issues (where you have to manually disable CF/SLI to even get a game to run), and microstutter. CF/SLI will also almost always run hotter and louder than a single card.
If you can find a steal on a low-end card the performance can be worth the headache, but at normal prices I'd just get a 6950.