Look into build threads here. There a few people who have written up comprehensive details of their upgrades (I have a thread about my IS as well.) Once you do that, you can devise a plan that comports with your parameters, or ask specifics that will get generate constructive responses (as opposed to a generic "what should I do."
Filipo, jtsny, and NoQuarters's threads/posts have helped me shape my plan.
Good luck.
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