okay mate. This is the go.
Normally Nissans run about 35psi of fuel pressure through the fuel rail at idle which then raises 20psi under boost. If you install a adjustable fuel pressure regulator you can set the base pressure to 55psi which then raises the fuel pressure to 75psi under boost which will squirt more fuel into the cyl and allow you to get that little more out of your injectors - think about a 15 - 20% increase in injectors.
So if you DO decide to do this instead of updating your injectors please go for a good quality fuel regulator.
SX
Nizmo
Sard
In that order - or any of the above fuel pressure regulators are reliable. The malpassi ones available have been known to fail so try and stay away from them.
If your only after a little extra power or keep the injectory duty down the fuel pressure reg is the way to go - otherwise get 700cc injectors and be done with it. Not worth exploding your engine.