I can't really answer the question in as much detail as it deserves but what I can do is point you to a site that helped me out a lot when it came to building out a spec list for a clients computer.
A great guide and parts list to building your own computer at various Price Points
Pick a price point and either use what he chose or use it as a guideline for a computer that fits you.
From what I've seen of his site, he goes much more into detail than he did before and I'm very impressed. Make sure that you upgrade and get something that is USB 3.0 compatible if you're planning on keeping the computer for some years.