Mattress buying help

Hi all,

My young friend is moving out on her own (and in with her boyfriend) for the very first time and has enlisted my aid in budgeting & finding the best deals on stuff they'll need for their home.

Currently my goal is to find them the best deal for a decent quality mattress & box spring. The only parameters I've been given are queen size, pillow top, medium firmness.

Problem is, I haven't had to buy a mattress in ages and am not sure what all I need to consider or what brands are considered good. They're planning on using their tax refunds, so I'm guessing the budget will be somewhere in the $500-700 area.

I'm planning on shopping around local furniture stores as well as online for best deals & maximum information. But I'd be remiss in my duties if I didn't ask you lovlies for any advice you can give me.