Well, first off you should start out by getting a ride in a 400rwhp car. I understand the desire to shoot higher, but 400hp is a lot of power on the road. Also, the twin setup will require things like an aftermarket K member and a LOT of work and $$. Also, to build a fuel system to support over 500rwhp, you start looking into thousands of $$... so it adds up quickly.
Best bang for your buck would be the single turbo systems. On a stock motor you can normally run 450rwhp with no issues. Contrary to popular belief, twins are no better than a properly sized single turbo, and tend to be more reliable due to having half the parts. A turbo will give you the widest torque curve out of the other forced induction options, and also produces about 80-120 extra usable HP at the crank because you arn't sucking that power off to turn a SC.
Remember to save $$ for brake upgrades. What goes, needs to stop too.
You will also need wheel and tire upgrades to keep that power on the road safely. Then there will be the random parts you will break/wear out that you should keep in mind as well.
For a twin setup, mild built motor, and supporting mods, your looking at an easy $15-20K assuming you can do some or most of the work.
Good luck with whatever you do.