I would start by looking into the Samsung lines. They look great, and come in a large range of prices. I have lots of happy clients using monitors from their Professional line, I think the previous monitors from Apple (LCD, not LED) were Samsung displays.
What kind of work do you want to do on the monitor? Do you need a particular resolution or refresh rate?
Since it's connected to a Mac, I'm guessing graphics or photo retouch might be of interest and for that stuff, and without giving up gaming or other uses which require fast response times, the Dell U2410 is the entry point. The Dell lists for $599 but every 2 or 3 weeks there are specials and I have seen it as low as mid $400's. Watch THG for specials as well as sites like fatwallet and techbargains.