Well, 15FPS in BC's cities does not sound that strange to me, now with a 2.0GHz processor. The weight on your setup in the cities are the AI packages, the CPU killers. I was happy with my FPS at 18 - 22 FPS with BC (no FPS patches), QTP3R, HD 4870 1GB and 3.16GHz Core2Duo. 1Ghz difference in the CPU speed is huge. That is your bottleneck. Not loading NPCs means less AI packages. In Bravil, Bruma behind the chapel and Leyawiin facing the city center, my FPS drop to the mid teens. PyFFI your meshes, but I would not expect FPS to increase to much with that serious CPU bottleneck.
Ah, and there's no way to make Oblivion run my dual cores? That's disappointing, but thank you for your help.
If you have onboard sound..Try Quiet Feet Mod
Also have you Tried OSR. Oblivion Studder Remover.
With the above 2 Mods I see great improvements.
What Resolution are you Using. Try Lowering to see if you get a noticeable Change.
http://sites.google.com/site/oblivionpoinfo/stabilization/fpssmoothing
http://sites.google.com/site/oblivionpoinfo/stabilization/fpssmoothing
Thank you both for the mod suggestions. I'm running 1366x768 as that is my PCs native resolution. I would more quickly lower texture, mesh and shadow quality then resolution, personally.
I wouldn't worry about it if it's just Waterfront and the Market district that makes your FPS fail, I have the same problem with those spots (Especially Market District is funny; depending on which direction I face my FPS varies between 10 - 30+). Better Cities cranks up the demands too high for you to expect it to run at 30 FPS all the time. Ofc, if you got the same problem everywhere it's not so good, but for me, even if my FPS goes down to around 15 in the Market District, it's usually around 60 when I'm out of the cities.
I can't recall having hit 60 since I installed FCOM on this computer :/ or 50 for that matter, in fact 40 is rare, haha... ah well.
See the PyFFI thread linked in my sig for information on PyFFI and where to download some already PyFFI-optimized mesh patches.
Thanks for the links, mate

Definitively, though, is there now way to let Oblivion use my full processor? Either way, thanks all for your help!
CC