I can put together a good $600 build for you, but in order to make smart choices on what parts to use I have to know what your goals are.
So, a few questions:
■What sorts of things do you use your computer for when you do use it?
■When you say you want it to be upgradable, is that to prevent it from being slow, or another reason?
■Do you care what the case looks like?
■What are you trying to accomplish with the i5?