Do you have to use a subwoofer in a home theater system?

I bought the components for a quality home theater system a few years ago. I bought a Hitachi plasma TV and all Yamaha pieces including a HTR 5990 receiver, a pair of 777 front speakers, a pair of 333 surround speakers, and a C444 center speaker. I think it was all top end stuff, I shopped around and ended up paying about $3500 for it all on ebay. But I never bought a sobwoofer. The system sounds great, but I was wondering would it make a big difference to add a good quality subwoofer? A top of the line Yamaha subwoofer is about $250. How much difference would it make?