If you already have an MPFI car (i.e. Si) then chipping the ECU and using an rpm activated switch would be the cheapest. (Actually, just using an rpm activated switch with a stock PM6 Si ECU would be the cheapest, but im talking *real* solutions here)
converting to OBD1 would mean buying a P28 ECU, buying an OBD1 Z6 distributor, OBD1 injectors, and all the plugs that go with it.
Like i said, its all personal preference.
BTW, MSD RPM Activated Switch (PN 8950) is a switch that you hook up to the VTEC solonoid via a 30 amp relay and it takes "pills" or modules labeled with a certain rpm. when the engine hits that rpm, it activates the VTEC solonoid. A VTEC controller (Fields, Apex'i, etc.) will use an electronic circuit to activate VTEC and you can also tune your fuel with it.