If you'd had it over 120, it may already be chipped. Or your speedo sucks.
Don't buy some random chip, get someone to make a program for it. The installation is fairly cheap, i used to charge $65 for socketing and a custom program but I don't have time to mess with much of that any more. There are local people who can help you out with that though. How easy is is to make a custom program depends on whether it's an OBD0 or OBD1 PR3 though, the OBD1 PW0 and PR3 are cousins, and both are bastard children, but the OBD1 PR3 is pretty much identical to the US P30s from del sols, etc.