The initial loading of Octane scene data will take a little longer via a 'slower' bus but for all intents and purposes it doesnt make a difference to the user experience.
I am not sure you will actually notice anything else going on around you anyway if you have 5 monitors to look at...
This also applies to the usual video I/O for gaming and probably to Premiere GPU acceleration.
More important for Octane is if your want to load big scenes that fill the 2gb vram you will have at your disposal with the Quadro 4000 (the cards run the same scene data in parallel so vram isnt accumulative) is to have enough system ram so it doesnt page to disk while loading and 'voxelising'.
I would advocate for 12gb in your case because no doubt Premiere can make use of a generous space as well.
Btw you can use a 4x speed slot as well but this is not so desirable. Some mobo have 4x slots or reduce speed to 4x because they share the bandwidth between slots if they are occupied.
It pays to read up on the specs even if you spy say 3 slots in a picture.
I am no expert but I am not aware of any user setting to control the bus speed. AFAIK this is auto handled by the mobo as a function of the technical design and the cards populating it.
Some expensive mobo do have an extra controller grafted on that allows more PCI-E slots to run at 16x but this is not a must have.
Soon, maybe next generation, graphics and mobo will come out with PCI-E 3 which will potentially double the bus bandwidth. This will be backward compatible - ie a PCI-E 2 card will work in it.
While every speed up is nice to have I am not sure it will seriously disadvantage anyone still using Octane at PCI-E 2 speeds.
I am not aware of any slowing of other controllers that handle the usb or sata.
I dont think there is any practical issue to worry about with PCI_E bandwidth for the mobo you have or other stuff being affected.
I dont have a lot of time to answer ongoing questions but I hope what I have written is helpful to you.
The most important thing for you is to check out the ins and outs of the 'Mercury' engine re 2 cards.
later
