Dual Opteron systems and the new AM2 chips have significantly more memory bandwidth than older chips. The soon-to-be-realeased conroe may be to your liking too, with more cache and possibly more memory b/w. that is surprising, considering that in most instances a dual-operton platform is considered to be superior to a notebook.
from what I know about HDDs, they are much more efficient if you can commit larger amount of data at once (sequential write) rather than sub 512B chunks.
there is, it's called perfmon, assuming that you are on windows AND using system calls for HDD access it should be able to help you. Alternatively, you could run it on a virtual machine just to analyze the process better.
indeed that should not be a problem, but depending on the access pattern the "average" speed may have little reflection what speed the HDD needs to support at certain instances.
well, if that is your goal, then you should take the person's advice and profile the program execution. While the profiling run will take longer, due to the overhead, it will tell you where your program is spending the time, which may be useful in determining not only the bottleneck, but will provide a starting point for future improvements as well.