I highly reccomend Tempur matresses. Since I've been sleeping on it, I no longer wake up in pain, as it supports and cradles the body in all the right places.
Unfortunately, they are costly. Our Superking size Tempur cost us �1400, but there are imitation memory foam matresses, which are still expensive, but cheaper than Tempur, around �300 to �500.
You spend probably a third of your life sleeping, so it really is important to have a good quality matress. You really can't afford to skimp on price, a decent night sleep is paramount.