i've used the code in the Article 20# on Nehe's site in my engine (dot3 bump map) , and works fine, but it applies only diffuse light. How should i set the opengl extensions in order to get also specular light? is it possible to do this in only one rendering pass ?

Well, it once again depends on the HW you are targeting. If you are using just TenEnv/Dot3 then I doubt you can get high enough specular exponent in one pass to be worth it. But it can be done using NV_RC/ATI_TS with somehow low exponent (16, maybe 32 if you sacrafise something else). On any more powerfull HW (using ARB_FP or GLSL) implemeting specular is a no-brainer :).

and so? how should i set register combiners?

Read the excellent bump mapping article at NVIDIA which explains everything from math equations of bump mapping to register combiner implementation.
It's one of the best papers you can ever find on the Web : clear, complete, and source code included !
But you are warned, it's not a very easy paper. You need to have some math background as well as OpenGL programming.