To my knowledge, a used 5850 or 5830 on eBay (or here on bitcointalk) will probably be the most cost effective as far as mhash/dollar. I haven't used a 5830 but I use a 5850 and some of them have great overclock potential (900 mhz almost guaranteed it seems).
The most power efficient GPUs are easily the 5970 (dual GPU card, best to buy used as it's rarely sold new now) or the new 7970.
5850s seem the cheapest $/hash when you get them second hand off ebay for around $100
Ive seen some great results from the 7970s especially when undervolted slightly.