We sprung for a new mattress about a year ago.  Went with a gel infused memory foam mattress.  A big thick 13" one that doesn't require springs or a platform below.  It sits right on the frame/foundation.  Was about $650 shipped to my door for a king size.  It came vacuum packed in a big baggie in a box about the size of a water heater box.  Lay it on the frame, cut the bag, and you've got a bed.  We are 100% happy

Here's the one we got:

http://www.samsclub.com/sams/night-therapy-elite-13-mygel-prestige-memory-foam-mattress-king/prod6480113.ip?searchTerm=null