Same problem here - Nehalem 8-core 2.26 16GB. After a reboot things are generally ok, however I have found that certain Logic projects are susceptible to audio engine overloads after a period of time working on them. In fact I have noticed that the Logic project file sizes increase steadily each time I save - one 20 min cue ended up as 100MB last week, I couldn't hit play without the audio collapsing instantly. But then on a new save the size will drop to 50MB and it's playable again. Other big 16GB-maxed projects run without a hitch.
Looking at Logic CPU monitor all the overload activity is on the last 2 cores, however in Activity Monitor both 64 and 32 bit VE Pro instances remain at 100 - 120% even while Logic is stopped. Logic remains at around 50%. This is without a software instrument track as "midi thru" record enabled. I'm using around 8 frames of VSL, Kontakt 3.5 (4.1 upgrade tomorrow) and Omnishpere, with quite a few Altiverbs, EXSs, Plays etc in Logic.
Would sure love to find a checklist of buffer / settings / guidelines for getting a big template to run smoothly in Logic with VE Pro.