Hey there,
I won't be attending Rice this fall, but I am a current Rice undergrad. As far as having a car goes, you can get by without one but it certainly wont be easy. The MetroRail only goes in one straight line across Houston. It's great for getting from campus to downtown, but it's difficult for running errands and such. There is a grocery store on one of the stops, but I'm not sure how much you'd want to cart your groceries back to campus. I'm not super familiar with the bus system, but I guess its okay. Are you planning on living in the grad apartments? The ones in the village are in a great location and you would have access to lots of restaurants, shops, etc. And there's a shuttle to campus throughout the day (you can also walk if you don't mind a 15+ minute walk). So in conclusion, you can deal without a car, but I strongly recommend having one if at all possible. Or find a roommate with one, that's pretty effective too.