I'm assuming you have an A1200?
If you want to stick with OS3.x, then you can get a Mediator and a 68k/PPC accelerator. This will also allow you to run MorphOS on your Amiga.
If you want to run MorphOS a lot faster get a Pegasos, but then you lose classic compatibility unless you use UAE on the Peg.
If you really want OS4 then you will have to try to get hold of a second hand A1, or wait and see if any upgrades for the A1200 are released which will let you run OS4 on it.
--
moto
--EDIT
Mediator isn't compatible with MorphOS. If you want to run MorphOS on a classic Amiga then you need a G-Rex.