Mkay, well there are a few things that you can change or add to make your computer a little faster.
Keep in mind though, that it isn't going to make amazing improvements. It will be much better, but not amazingly better.
First thing you need to add is more RAM (or memory). You really need to have 3GB in your computer. I'd recommend 4GB, but your computer is limited to taking just over 3GB (so adding more is a waste of money).
Second thing is getting a graphics card. The more money you can spend on it the better - within reason. I wouldn't spend more than $200 on it, because any more than that, and you won't be able to use the full power of the graphics card - you will be limited by other parts.
Third, if you are getting the graphics card, get a new power supply. Go for a decently priced one - not cheap. Aim for around the 450w-550w area. Any more than that is completely unnecessary. However, you'd generally be better off paying $50 for a 450w, than $20 for a 550w. Better features in general and less risk of the power supply ruining the computer (or exploding).
This site might be able to help you.
http://www.tomshardware.com/reviews/best-graphics-card-radeon-hd-6990-geforce-gtx-590,2912.html