Kurse, you are asking the right questions, but you may get some bad advice here, evaluate it carefully.
A second drive for backup is a good idea if you have data that you want to protect. I would, however make it an external drive, connected by USB or E-sata. Back up to it as often as you need to, disconnect it, and put it in a safe place. Data can be lost not only from hardware failure(rare), but more commonly by virus, or inadvertent operator error. Raid-1(mirroring) can protect against hardware failures of certain types, but not all loss reasons. External backup is best.
For the single user desktop environment, the WD 150gb raptor is currently the best performer, although some of the newer 750gb units are not far behind. Go to www.storagereview.com to read up on hard drive performance. If you have a few extra dollars to spend, I would get a 150gb raptor, and when your capacity needs exceed 150gb, then add a second drive for overflow storage.
Partitioning is not a performance enhancer. It is best used to add a second logical drive so that you could dual boot a second os. If all your needs fit on one drive, I think that is the simplest, cheapest, and best solution. If your work is of the type that reads one file sequentially, processes it, and writes it to a second file, then two physical drives is very good. Put your input on one, and your output on the other. This minimizes arm motion. Putting the OS on it's own drive was somewhat helpful with windows XP. XP tried to conserve a then expensive resource(real memory) by writing it out in advance to the page file. Vista, on the other hand, tries to KEEP as much useful stuff in main memory, so that it can be used instantly. Since memory is now much cheaper, this is a better strategy today.
Some here will suggest raid-0 as a performance enhancer. It has been shown to have almost no benefit in typical user applications(vs. synthetic benchmarks). In certain sequential operations, it might, but even then it is better to use two independent drives.
In the end, with the technology of today, there is not a very large difference, no matter how you do it. When we finally get affordable, high performance SSD's, then you will see a very noticeable difference.
---good luck---