1998 M52B28 engines have OBDII emissions. They do not have a chip that you replace, but rather software that you can download.
You don't see much of a gain from a chip on an OBDII car, at least not when compared to OBDI.
I would think you are better off going with an intake and exhaust, as well as perhaps the Schrick intake manifold. Those may require custom software once you are done with all of the Mods.
If you do go the software route, Jim Conforti (Shark Injector) probably makes the most popular software. Dinan is OK too, but most people seem to believe Conforti gets a little more out of the engine. I had a Dinan chip in my 325i which actually gave a good increase, but that was an OBDI engine.
You can get Conforti software from Conforti directly or Turner Motorsport (and I am sure other places as well).