Ill keep this short and sweet. Just buy properly sized injectors for your power goals. Buy a good quality brand, like Injector Dynamics, and have them get set up by a good tuner. Don't over think it. Buy new O rings too.

OEM RB20DET = Top feed High impedance injectors. Stock is 270cc. Each injector will pass on about 10 ohms. Each is wired in parallel so that the total return to the ECU is ~10 ohms.

OEM RB26DETT = Top feed Low impedance injectors. Stock is 440cc. Each injector will pass on about ~ 2 ohms. Each injector is wired in parallel to the ECU but in Series with a resistor. That resistor passes on about 7-8 ohms. The two signals add up to ~ 10 ohms returned to make the ECU happy.

Click Here For An Injector Size Calculation Guide