I'm going to ride my Giant Boulder too! (2004 model)
Have just fitted some Specialized Anaconda 26 x 1.50's after training for months on the nobblies. These new ones are highly puncture resistant semi slicks that you can pump up to 80psi.
Have found them excellent so far. Really makes a difference to cruising speed and effort required! Also haven't had to fix a puncture since I've had them. (Had had a fair few in the weeks prior with all the glass, wire etc on the road and shared paths around my way.)
My slow haul up Bedfordale hill is now that little bit faster and not quite so tiring. (I think the red tyre walls really do make you go faster! Maybe I should paint my bike so I can keep up with some of you guys.)