First, What do you mean by the second core isn't used. Like most programs don't use it yet, or it's just not working?
Second, assuming you mean the second is just not utilized much, if you're concerned mostly with gaming, you might be better off saving your money, and buying a DX10 card when DX10 finally releases. If, however, you plan on overclocking, and you also do things such as video rendering/encoding, or anything else cpu intensive, the Core2s show a huge performance increase given similar clock speeds. If I remember correctly the performance delta starts at 25% higher and goes all the way up to 70% higher(depends on the task) in almost everything except gaming(which does show an improvement, just not as much as other tasks.)