Enroute Controller described a NMAC between an IFR and a VFR aircraft; claiming the facility practice of combining positions that include ground to FL350 altitude responsibilities is unsafe

Narrative

I was working a high altitude sector that owns FL350 and above. I witnessed a near mid-air collision between a BE24 level at 5;000 and unknown VFR aircraft. The traffic volume and complexity of owning from the surface to FL340 is ridiculous! On any given day sectors are combined 95% of the time with low altitude airspace open. So de-combining the high altitude sectors is not the answer to the problem. The answer is to keep the low altitude sector open. If the low altitude sector had been open the 5 aircraft in their airspace at the time the pilot of the BE24 would have had better service! The BE24 descended and turned right to avoid the traffic. With the approach being open more efficient traffic calls would have been made with a better RADAR servicing the airspace. The Approach RADAR can see to about 800 FT above the subject airport. Our Center RADAR on a good day will pick up aircraft about 3;500 FT. Recommendation; keep the low altitude sector open for at least the normal weekend hours during the holiday season.