the only downside I find with raid is, using small drives can largely limit your performance. Multi platter drives just perform better, and going for at minimum the single platter version of the highest platter density is the best way to go.
80/160gb drives often have cheap platters in that don't perform up to scratch of a higher density platter, and multi platter drives tend to always have faster access times as the drive can access more files at a time. IE, going to low kills quite a bit of performance making raiding them a less effective.
I tend to try and buy 2 of the newest drives for a new build to raid but while you don't want to buy say 1tb drives to raid, the lowest aren't the best option either.
I'm considering what drives to get at the moment for a new Phenom 2 build in a few days when available, it would seem maybe Samsung 640gb's with 2 of their biggest platters might be the cheapest/fastest setup without going for raptors/ssd's and their increase in costs aswell. Theres supposedly a new 500gb Seagate single platter drive that "could" be insanely fast and not bad value either.
the price increase for capacity from lowest to midrange drives makes midsized drives much better value.
lol, basically, I have no idea what to get right now but go to small and to far down the range and two raided won't be much better than a better single higher end drive.
Been looking up hard drive reviews today and, its very hard to find good comparison sites, even less of the sites there are really go into raid performance and even less of those reviews do raid across the range of drives around, just on a couple top end expensive drives.
For instance 1x 160gb spinpoint is around £30, and £48 gets you the 640gb version which is supposed to have 2 of their highest density platters in, same as the 1tb drive, which is pretty damn nippy. 4times the storage for less than 50% cost increase and much more performance aswell.