Assuming you use the same graphiscs card:
A dual core CPU runing at 3.5Ghz will play Oblivion faster than a quad core CPU running at 2.6Ghz?
As I understand correctly, the lag is due to the CPU processing AI scripts (actors, lights, objects) and not textures VRAM usage.
Am I correct?
On another note:
As for textures, in my 140 hour gaming experience (using ATI 4870 1GB VRAM) over the past three weeks I discovered that:
Running QTP3 full and seeing VRAM consistently at 800MB outside (give or take 300MB at times) compared to running Vibrant textures and seeing VRAM consistently around 400MB consitently made no noticable difference in my game when outside. The lag when actors and lights were in play was the same - bad. I did find the vibrant textures are really nice, especially in dungeons and ruins. I finally settled on a combination of both.