The Rockets are starting 6'6" Chuck Hayes at center. While he is one of the better post defenders in the league, it's hard to be incredibly effective at that height.
Luckily for Houston, plenty of quality free-agent centers will be available this offseason.
Marc Gasol is my favorite option and the Grizz are going to have to decide between him and Zach Randolph. They'll probably choose Gasol, but if Memphis chooses Randolph, Gasol would fit in magically with Houston.
Standing at 7'1", Gasol would make an immediate impact defensively to go along with solid offensive and rebounding skills.
DeAndre Jordan is another option; he is young, a great shot blocker and extremely athletic. He is a terrible free-throw shooter but it's more than than likely he'll be available depending on the Clippers' interest in re-signing him. But I don't think it is on the top of their to-do list.
Tyson Chandler will more than likely be re-signed by Dallas, but if he isn't, Houston would be knocking at his door the moment the clock strikes midnight and free agents can be negotiated with.
Samuel Dalmbert would be an inexpensive option and I wouldn't be surprised if the Rockets end up signing him. He is a very good defensive center, and like I said, he will not be very expensive.
Nene Hilario's decision on his player option is also pending. If he goes the free-agent route, you can rest assured Houston will be in contact, as they previously tried to swing a deal for Nene at the trade deadline. It's obvious the Rockets have interest in him.