On MAC's I think it depends on Software: Logic 8 and my 2.4 Duo (logic software shows it's using 2 processors or cores)
Logic 9.1.3 (64 bit) and latest update of SnowLeopard with Imac I7 Quad, Logic shows 8 cores avail due to hyper-threading and I can assign how many cores I want Logic to use.
So If I set so Logic can only use 6 cores, it free's up 2 cores to run virtual instruments........This only works in 64 bit mode, and I can switch to 32 or 64.