Auto-detecting and loading OGL dll

I’m coding in opengl using SDL. And i want my programs to be 3d accelerated also on 3dfx cards. How can i do it without overwriting opengl32.dll with 3dfxvgl.dll?
I also want to make a menu where you can choose between the opengl drivers.
I’m new at windows programming - that’s why i’m using sdl

Sorry for my terrible english