How can a computer have 3gigs of RAM?

Never seen a computer that required matching pairs. 3 GB is a marketing gimmick. A lot of consumers do not understand the benefit of dual channel memory but believe that bigger is always better. For general usage, a computer with two matching 1 GB memory modules is faster than one with size mismatched memory modules which will drop the computer into single channel mode. Your best bet is to use a utility like CPU-Z and purchase a 2 GB module which has the same memory timing as the one you already have. In some cases, it may make more sense to buy a brand new pair of memory modules as it is sometimes difficult and costly to find a match for what you have based on its age.