|Great answers, but no-one actually addresses the question that the OP asked. Can he mix memory modules of different sizes in his computer?|
The short answer is yes you can, but you would be advised not to. The best performance is achieved with matched modules and memory is very cheap at the moment. I would advise getting one of the 4GB kits from Crucial and replacing the existing 1GB. As you are running Windows 7 you should see an appreciable increase in performance.