I figure it's worth echoing what ERP said prior to anything else, but to answer your questions... It again seems to be the case with any CPU. We're not really multithreading anything on the PC, and even the extent of work we have in that respect on 360 is pretty localized to things like audio, physics, and AI. So on the PC we have hell whether it's a dual-core A64 or a single-core P4. Bear in mind that neither of these projects are likely to be out until mid-2008. In a funny way, in spite of having a very incomplete codebase for the PS3, there are already enough SPE-directed tasks to give it something of an edge over the state of code on other platforms -- even though the only SPE code we have at the moment are the really obvious candidates.
Assuming that we can ultimately get around to making more cores fly on the 360 and PS3, and we follow in kind on the PC, you can theoretically see a benefit over 4 or 6 or 8 cores, but I can't say it'll be terrific or horrific... We'll cross that bridge when we come to it.
Though there are definitely cases where raw CPU power just for our end of the code is a problem on the PC but not on the consoles, but they come down mainly to just throwing a lot of activity into a scene.