I totally disagree with Silvercoord..........not really but it is fun just to disagree with him for the heck of it. He is the poster boy for OpenGL here and I guess I'm becoming a poster boy for Direct3D...and, of course, assembly. He has challenged me in many areas which I thoroughly enjoy and have learned much from.
Go get some books on 3D math. Not the textbook college ones because they are explained by people who could make tieing your shoe into rocket science. Most modern 3D math books are written with you and I in mind and are extremely good sources of information.
By the way, Outcast 1 is a far cry from Quake 3. Outcast 1 used a voxel engine and voxel texturing. The objects were rendered as though the camera was looking down on top of them - for more information you can do a google on it and there is a huge site which talks about it. Unfortunately Outcast 1 is a ........ poor example of a voxel engine - Nova Logic's Voxel Space 1,2,3 (Comanche series, Delta Force 1 and 2, Armored Fist 1 and 2, etc) are much better examples of very very good voxel engines with 6, yes count 'em 6 degrees of freedom.
Anyways as to which API is better is like whether or not turkey is better than ham. It's purely preference. Personally I like turkey so I'm definitely a DirectX dude.
Welcome to the exciting world of game programming - there are no limits.