Keeping u`r windows install on a seperate drive to other stuff u work on, games or whatever, will always give u a drive performance increase - just on the logic of moving parts not having to move to much.
What sort of noticable difference will depend on what drive (and type) u start off with and what drive u add. These SATA2 drives, just cos of the average low-usage nature of a home user, will often show very very little difference.
Sometimes the difference can be very very subtle, and if u`r spending bucks on it - it may not seem worth it. But just as an example, if u ran programs to convert large video files and like to use the pc to do other stuff whilst it`s working, the drive will be interfered with less if u have 2 and u`ll notice the time difference, similarly with dual cores.
Imo, the pricing of a raptor 36g sata1 just aint worth it.