Put most simply I think grieve said it the best. Just so that you understand though, there are multiple factors that go into each piece of hardware that make it better than another piece.
For example, GPUs have both speed an memory factors that you need to take into account when shopping for one. CPUs have speed, number of cores, and front side bus speeds that highly affect it. Finally, memory has speed and obviously, sheer numbers (4gb>2gb).
If you are looking for an upgrade, the easiest way to do it is by keeping in mind the following things:
1. Is it compatible?
2. Is it coming from a respected company (ex. Asus)
3. Is it in my price range? (If you don't know a lot about PCs, one of the easiest way to figure out if you are getting a quality item, is how it stacks up to the rest in its category as far as price goes)
Also, I would like to mention, that for upgrade purposes, RAM (memory) should probably be your first thing to look into. Often times it can be an easy and affordable upgrade, and will spawn some great rises in your FPS.