For your first mattress and box spring, go to the store and talk to the guy about the differences and test some out.
I like my Serta. I know it is an old standard company, but it is comfy and hits just the right point of firm and soft.
I dont like the memory foam beds... I SWEAT like a cheerleader in a locker room. I am hot natured and those mattresses make me so hot it is uncomfortable. But I am a hot natured dude.
Buy quality. This is one thing for your house that you get what you pay for. A cheap mattress/box springs combo will not last and will sleep like a cheap set. If you buy outrageously expensive, you may be buying more than you need.
I would seriously advise to go to the furniture store, (or Sears) and just test them out.