Before I say anything, I do want to give a special thanks to Davinleeds. I hope you didn't misinterpret my previous post as anger toward you. My frustration was with the situation, and I greatly appreciate the suggestions you offered...
I now have smooth BD playback at both 1080p/24 and 1080p/60. I am not overclocking, and to push my luck, I enabled Cool 'n' Quiet. So far, playback of h.264 material is silky smooth.
Reminder of my setup:
ASUS M3N78-VM (Geforce 8200)
CORSAIR XMS2 (2 x 2GB) 240-Pin DDR2 SDRAM DDR2 800
LITE-ON BD-ROM iHOS104-06 (4x)
The Asus tech told me to pull one of my memory sticks. Wham. All of a sudden, smooth playback. I was always under the impression that for best performance memory should be installed in pairs. Apparently in my case, a single 2GB stick is best. (I'll save the other for that WHS server that I want to build.) I didn't totally understand his explanation (language barrier) but he said something to the effect that memory utilization was not as efficient as it should be with multiple sticks.
I hope this helps anyone else that is having problems. I'll probably hold on to the Radeon 4350 I ordered for a couple of weeks, but everything looks great at the moment...
Thanks again to Davinleeds!