So I built I budget pc for my mother. i540 and a (1)WD6400 blue. This thing boots up way faster than my RAID 0, WD2500JS i7920. Like Noticeably faster boot up. I ran the HDtune read and write times were about the same. On both machines, around 100 mbps. My hard drives are about 3-4 years old, and the specs look pretty much the same, as much as I can tell. Are newer drives somehow faster on boot up? It seems like I really don't need to raid my drives anymore. Both are SATA connects. Single drive is looking pretty sweet.
If you haven't loaded the same software that you run on your mom's new build, it is basically booting windows, with no pre-fetches of all the other software. It'll slow down a bit after you install office, another web browser, CD/DVD burning software, etc.
But your point about RAID is a good one. When the RAID works, it is good. I've been having some trouble with my two-year old RAID 1. One or both HDDs keep randomly falling out, requiring frequent re-builds.