I learned all the needed stuff in GL that in need. After that I did a little performance test:
Remember that when I use SDL and blit a 640x480 background it uses 50 - 60% of my CPU.
So I did the following in OpenGL. First a 3D cube is in the middle of the screen (screen is 800x600) that is textured with an image and rotates on many dirrections. Then the background is textured with a big image. Then there are many textured boxes (100x100) drawn on the screen, all textured). And the whole screen in rotating.
In the first test I created 50 boxes every frame and textured them with a 640x480 image. That means that a 100x100 place was filled with a 640x480 image. The CPU usage was from 2 - 10%.
(Open the image in new tab/window to see if fully).
Then I thought about it and figgured out that texturing a 100x100 rect with a 640x480 image isn't realy performance efficient and you don't get a better image. So I resized the image to 100x100 and then textured teh rects with it. But now I created 200 rects every frame. And what came out: 0% CPU ussage :D:D Yep thats great. :D
Conclusion: OpenGL is great. Now I need to get an idea for my next game :D