I'm learning Houdini and for that I'm creating a little picture, a wizard in his lab'

I come from MentalRay so I have few questions… so of course the picture is still very rough but I have already a quite long rendering time … around 25 minutes in 400x600. So, for this kind of picture I made the scene with a micropolygon rendering, a low level sampling and a GI shader with path tracing.
I have also a lot of raytrace shaders and some displacement shaders. Is it normal to have this rendering time ? It's sound to me that it's too long, what can I do for reduce it ? With the raytrace engine mantra just doesn't begin the rendering !
my PC : intel quad 2.66 GHZ 3Go RAM…
It also seams that I have a lot of nose but not everywhere ! It's look like just shadows are noisy but I haven't any area spot … ( just normal spotLight and two Omni )
Also any idea for these black spot on the Orb ? ( in a Frosted-Glass shader … )
Outch a lot of questions sorry… Thank you very much ! and sorry for my English too…
C.