The differences are:

Base and Civic Si: 2.0L engine with iVTEC. It only has two intake lobes. VTEC only works on the intake side. The engine has a compression ratio of 9.8:1. The engine uses a two stage plastic intake manifold. Long intake runners for low-RPM , shorter runners for higher RPM

RSX Type S: 2.0L engine with iVTEC. It has three lobes. VTEC works on both the intake and exhaust side. The engine has stronger connecting rods because of a compression ratio of 11.0:1.

Hope that helps!

~Nightcrawler~