Linux doesn't dump memory of programs just cause you closed them, it maximizes your RAM by keeping what i feels necessary in Memory so the next time you open it, it will open up faster and won't have to reload itself into memory.. If your not going to use all 512MB of RAM.. why have that much RAM? Try searching on this subject, its asked all the time.. literally.. all the time. Remember, Linux isn't Windows and doesn't have crap RAM utilization like Windows has..
And if your system is slower, most likely is caused by other issues..