There are so many variables involved in this that it's difficult to give a succinct answer. Best searching this forum, there's plenty of good advice on partitioning (also over at <A HREF="http://www.storagereview.com" target="_new">http://www.storagereview.com</A> However, in response to your example:
Depends on what you had on each partition. However, generally speaking, if you had say a 120gb h/d split into two 60gb partitions, with winXP on the first and data on the second, speed would decrease marginally as the heads have to move from one partition to the other to read data and then OS files. However, the difference may not even be noticeable, the advantage is that you keep your data organised better, and you can backup more easily. Of course, if you put the swap file on a seperate partition (on the same drive) to the OS you'd lose speed for no benefit.