There are different kinds of foam toppers.
I dislike memory foam, would not have one. It is heavy, hot and stays mashed down.
I don't think it supports like a latex topper does. But it seems to be what the foam companies are pushing.
I like a latex foam topper at least 3 inches. I ordered one online that came with its own cover and skirt to fit on the mattress like a regular mattress pad. It made a total difference on our mattress that seems more like a box spring than a mattress!
But everyone has their own preference, just find what suits you best.
There are lots of places online to order. I think I found best price at Overstock and their shipping is cheap!
2016 Wildwood 26TBSS
2012 F150 Screw XLT
4 leggers Spudz and Halley