That's a really broad question and this probably isn't the right forum to ask that.
But the two main things we need to know is what is your budget and what is your goal? Are you wanting to upgrade just to upgrade? If so it's a waste of money. If you have a goal in mind we'll need to know. I have about $2500 in my fuel system, but I can support 800rwhp on E85 with no worries.