How can I make my new computer faster?

I just got a new computer. I am not completely computer illiterate or anything, but I would just like some advice. Its a Dell Inspiron desktop. It has 4 GB of RAM and a 640 GB Hard Drive and it has Windows 7 on it. When I first got it, it was very fast and of course since I have added things such as messengers and other things to it, it has slowed down a bit. It had Internet Explorer on it and I switched to Firefox. But I would just like to know if anyone has any tips when it comes to Windows 7. You see I expected with the amount of space I have on this computer it would run a lot faster, but it still seems to lag sometimes and I have only had it for a week. And even with all the programs I have added the hard drive has 521 GB of space left. The thing I am not so sure about is RAM. What exactly is that space used for? Right now that is at about 43%... which seems a bit high to me. Is that normal? Also, I am usually only running one or two of the programs at once. Anyway, any advice you have for me would be great! :)