I am wondering if I had made a mistake in buying 4 GB

Salutations and Greetings Ladies and Gentlemen, I bought a laptop that has VBB 4 GB PC2-5300 667 MHz 2DIMM but the laptop is only reading 3 GB. I was led to believe that buying the extra gigabyte would enable me to get 64–bit software. Is this true ? Based on the informative and helpful reply from Palmetto, this may not be a good idea. Is their a use of the 4 gigabyte with SBB Intel Core 2DUO Process T 7800 with Microsoft Vista, 32bit, Ultimate ? I am also planning, by virtual install if that is correct, on using Linux. Thank you. Ldxrunner