the best time to buy a mattress is on a holiday-- they have crazy labor day/memorial day/black friday sales for mattresses. we got ours for $500 less than the normal price on labor day!
we really like our mattress-- it has springs that are not all connected to each other, so my husband doesn't get jostled when i roll over in bed. i forget the term for it... there's some fancy name... and there is a memory foam top which is nice.
go to the showrooms and don't be embarrassed to lay down on the beds and try them out. ask lots of questions, the salesmen can help you figure out what kind of bed you need.
we saved some moola by skipping the boxspring and frame and putting our bed on a wooden platform instead... i think it makes our mattress a bit firmer than it would be with a boxspring but it works great for us!
also be aware that most beds these days require extra deep fitted sheets, so you might have to invest in those too. big lots and old time pottery are two places for discount prices on sheets...