The title of a game isn't copyrighted - it is trademarked. Trademark law and copyright law are related but entirely different areas.
While registration of a copyright is optional, a trademark must be registred to be enforceable (there are exceptions, like claiming an unregistred trademark through notoriety, but these are hard to prove in court).
Trademarks registred in the US can be researched on http://tmsearch.uspto.gov/ but keep in mind that trademarks always apply to individual countries. So when you sell a game internationally, you need to make sure that you don't advertise it in any country where the trademark is registred.
Trademarks always apply for specific groups of goods and services. So when I have a trademark "SuperAwesome2000" for toothpaste, kitchen appliances and sportswear, you could still name your game "SuperAwesome2000" without infringing my trademark, because entertainment software is a completely different field. You could even register the trademark yourself, as long as you do it for different product groups than I did. But when you start selling t-shirts advertising your game, we have a problem because you are infringing my trademark on sportswear.
Interesting read in this regard: Apple Corps vs. Apple Computers.
But like in every legal matter: When in doubt consult a lawyer.