I am glad you asked. Been looking for one. The issue is I can only take a single width card. So the question is, what can I get in single width, that will give me 150+, for under $150?
Oh yes, it also has to be a current production card.
Why can you only take single-width? Just buy a PCI-E extender cable and mount the card somewhere else in the case.
If you're going to force single-width, then just keep mining on nVidia. The double width cards are double-width because the heat generated by the GPU(s) requires additional cooling that a single-width card cannot accommodate. So the only single-width cards you're going to get are going to perform as poorly as your nVidia.
Better yet, just don't bother mining. You'd be better off taking your $150 and buying 10 BTC immediately at the exchange. It would take you months and months of mining on a single-width card to generate 10 BTC (probably closer to a year). Then wait for the price to go to $30 again, sell them, and then buy a nice rig.