Also, from what I can gather you've probably already got one of the fastest Skylines on this board. That speaks volumes really.
It's worth remembering also what is deemed as "practical" upgrades. Whilst it is conceivable (and has been done) to tune a Skyline all the way up to 1000bhp - the expense involved rapidly approaches (and goes beyond) that of the cost of the car itself. From conversations I have already had along the same lines, I would personally aim for around 550bhp tops - which is very expensive yet achievable, and more importantly - maintainable.
From what I can gather, the cost rises exponentially beyond that. So - if we're talking about £6000+ parts for a "reliable" 500bhp car, you're probably looking (at a rough guess) at £15k+ for 600bhp, maybe even more - you would have to ask people that are already approaching that.
Just my £0.02