Well, I don't know what kind of motherboard you got, but if it's a newer one(which supports dual cores), I'd suggest you to get these first. Gives you the most performance boost. Together with 4GB DDR2 RAM, it shouldn't cost more like 150-200$. You could also get a nice graphics card for around 150$. For the other components, I bet they ain't that urgent, since you already have them, right? Never the less, they shouldn't be more than 100$.
Also, what kind of power supply you have? We need more details, otherwise we can't really help, because we don't know what you already got.