Ideally you want 1" continuous air space under the roof sheathing and above any insulation. Modern truss packages use blocking between "rafters" with (3) 2" screened holes, every other rafter bay (24" o.c.), so if you use (3) 1" venting plugs (little aluminum grills that fit a 1" hole) every bay you would be fine. But they can't be covered by insulation.
The 2009 IRC specifies that the free venting area be 1/150 of the vented area, with a reduction to 1/300 if 50-80% of the venting is in the upper part of the area to vented (i.e. ridge vent), and the balance is at the eaves. My roofing supplier says they recommend 100 sq.in. vent per 300 sq. ft. of attic.
You can keep your gable end vents, they used to think it allowed the venting to "short circuit" but that's no longer the case. Hope this helps.