OpenGL newbie seeks help!

Hey /prog/, I'm writing a cross-platform OpenGL application which does some off-screen rendering. I've found two ways of doing that - with pbuffers and with FBOs, the latter is preferable. I want old videocards to be supported too.
Now tell me, which technique should I use? Do the older videocards support FBO? Do they support pbuffers?
Thanks.

>>20
First Quake did this all without any problem.
I'm no Quake expert but didn't they cut corners everywhere, most notably in the texture sampling?

Can you run Ruby in DOSBox, having a few gigabytes of memory for game world?
I don't know. Is there a Ruby port for DOS?