New Box Spring

Since Mary's question went unanswered, here's what Consumer Reports says --

Do you need a new box spring?

Foundations can sell for as much as the mattress, even though they're generally just a wood frame enclosing stiff wire and covered with fabric to match the mattress.
We found that companies frequently pair the same foundation with mattresses in different price ranges. You can save by buying a higher-priced mattress and a lower-priced foundation. Once the bed is made, no one will know. If the old box has bouncy springs instead of stiff wire, it should be replaced.
If your current foundation is only a few years old, with no rips, warps, creaks, or "give," consider using it with a new mattress. Though most respondents to our recent subscriber survey replaced their foundation with their mattress, roughly 80 percent of those who kept their old one reported that they were sleeping better after replacing just the mattress. So if your box spring isn't broken and is still structurally sound, consider keeping it and saving several hundred dollars.
If your new mattress is ultrathick, consider pairing it with a "low profile" foundation, 4 to 6 inches thick, to reduce height.