[shilala]

For whatever reason, I was in a waterbed for years. They rated between suck and not so bad.
10 or 15 years ago I finally couldn't take anymore and went shopping. It took months to find the time to hit all the different places and try all the different stuff. The first things to go were the gimmick matresses. The sleep number and aerospace engineered light years ahead mattresses were super uncomfortable, even though I'd set my mind on them.
Then I found a Sealy Eurotop double sided pillow top. It was sheer heaven, and it was priced as such. It was worth every dime.
When I got divorced I took another angle. I found the Sealy mattress I wanted, then shopped it on eBay. I saved over $500 on that deal, shipping included. I'm on that mattress right now and after 6 or 7 years, it's still heaven.
Like Jamie said, the bigger the better (ours is a king). I doubt you have that kind of room, but get as big as your budget allows. Consider the cost of bedding, too.
Try to stay with big name, proven companies. Get the info off the mattresses and from the salesmen and write it all down. Take it home and shop eBay. Don't be afraid to consider an imperfect, they may have a scuff or dirty spot that'll clean right up with Resolve. With eBay, some Resolve, and some elbow grease, you could probably save 50%, or afford twice the bed.