When I started looking into quad core applications, the main problem that I noticed was core usage. Oftentimes, users said that all 4 cores were being used by the program, but some were not being used 100% by the application. I heard that with sp1, vista will be able to support up to 48 cores. But my main concern is that vista 64 ultimate will only use each core partially. Do any of you guys have a quad core system with vista ultimate 64? If so, how is it going for you? Does vista do well when it comes to using quad core to the max, or is it going only halfway?
This is less of an OS issue than an app issue. Very seldom will a home user get into a situation where the OS has to use multiple cores (that is some serious CPU). Some apps and games are multi-core aware and can easily use more than one core, but very few home apps are. You have to narrow your question to what types of software you are running for determining how it uses your cores.
As PK said: That is a function of the Application, and not the Operating System. As an OS, Vista (XP, whatever) sees all of the CPU cores and can use them as needed. Vista is supposed to be more efficient/aggressive about it than XP, but in and of themselves the OS's simply do not need that much horsepower.
It is up to the application programmers to write multithreaded programs that can take advantage of multi-core CPU's. The OS can't magically force a program to take advantage.