Double check the foam weight of the backing foam as well. Some memory foam mattresses have too soft of a backing foam (non memory foam), so that can lead to major sinking into the bed and bad posture.
Memory foam also retains heat very easily. I have a window AC unit keeping the room at 70-72*F and have a fan blowing on me and I still get too hot at times. My matress also has air channels which help, but if you get hot easily while sleeping memory foam will just make it much worse.
Right now I have a memory foam mattress I got off Woot for about $300. Very comfortable, though I got it in winter. Great during that time for the body heat retention, but I'm finding it way too hot for summer use. That being said, I do get hot easily and generate a lot of body heat during the night. I'm still trading it in when my girlfriend moves in for her traditional mattress as I find I sleep at a much more comfortable temperature.
Other random notes: if you order one online, unpack it and let it air out/expand for a few days. It'll get rid of the smell, too. Also, you'll want to either use a solid platform for your bed (such as plates, plywood, etc), or keep your existing box springs. Memory foam matresses need something solid to sit on. Mine works fine on my old box springs.
If you do get one, I'll double coloumb's suggestion and buy one from a place that has a reputable return policy. They're not for everyone.