Getting iMac 27" Suggest good 2nd Monitor???

I'm going to be buying an iMac in about 6 months... I plan on using a 2nd monitor with it. My primary use - as a photographer - is Photoshop and Lightroom. I'm looking for suggestions for a 2nd monitor - I only have $500 or less to spend. Any suggestions from iMac users as to monitors I might look at and compare? Note: I'm currently using a PC and my external monitor is dying... so I'll need a monitor sooner than I can get the iMac.