A stable system is the result of careful planning and selection of components. I also wanted a stable system, and now I have one.
I chose a good, solid, fast CPU (AMD Athlon XP 2500+), a good set of memory (2x512MB Kingston), and a good motherboard (Gigabyte GA-7N400 Pro2). I put it in a nice new case (Antec P160) with a good, quiet PSU (Seasonic Super Tornado 300w).
I am running my OS, data, and apps on raid-1 (2xIBM 180GB 180GXP, hopefully to be replaced by 74GB Raptors when I can afford them). I Ghost my OS and important apps partitions (I have 2 apps partitions, the second for things like road atlas, medicine encyclopaedia, etc.) when things look good. The images (I keep 3 rolling versions) are stored on the raid-1 array itself (slows down the image process, but so what - it is only done once a month or and adds 1-2 minutes).
This protects me from disk failure (raid-1) and software/virus corruption (ghost image).
When I get the Raptiors I will make the 180GB drives into a raid-1 to store pictures and movies (which are currently on them anyway, but performance will be better when separated from apps and swao file).
Based on what you wrote, you are probably better off getting a large PSU (400w-500w+) and a couple of independent drives in addition to raid-1. You can then let Windows manage the swap files (created one on each drive) to increase peformance. You may also choose to go with raid-10 - two raid-1 arrays striped in raid-0. This will provide both performance and redundancy and is much faster than raid-5 with only 33% additional cost.