So I'm building a my first build and I'm not sure which CPU to get.
I want it to be something that will last ages and still be decent in the long run so that's why I'm thinking of the quad, for longevity. But surely I must be giving something up with them being the same price, like FSB, but what else?
Xp doesnt use more than 2 cores or virtual cpus effectively. Quad core is a waste on XP.
Sage though you are, that's not quite the case.
XP Pro (32 bit or 64 bit) is constrained to 2 Processors, each with as many cores as are designed into each CPU. These constraints are not present on the more expensive Server versions of XP. Same goes for Vista.
Indeed I know this empirically because the 4 cores in the Q6600 deliver their results in just under half the time of the Core 2 Duo.
Its to with how XP allocates timeslices. 2003 uses a different model.
There are differences between XP and Server 2003 on timeslice allocation. But that bites principally when you have to balance a stack of high demand background process with foreground processes (user applications).
In any case, you can vary the foreground or background priority in XP.
I repeat, the Q6600 scales performance fully where you have a multi-threaded application or one that can spread its load across CPUs. "Old" single threaded applications will be distributed across the 4 cores and that has to be better than 2 cores.
Of course, a single "old" single threaded application won't work any better on a dual core processor save for the fact that background processes have the other core available. With quad core it's obvious that the single threaded application has a better chance to run.