Kind of like building your own rifle from a Mauser action, it's really not cost-effective anymore.
I disagree.
With a name-brand computer, you're stuck with one (a few if you're lucky) chassis style, and it's usually terrible. Hard to open, hard to add components to, poor layout. I have a 6-year-old Dell Precision case where the floppy has been rendered just about useless because the little button on the front plate that's supposed to hit the eject button on the drive has gotten misaligned and/or bent. Fishing disks out of the floppy drive now involves taking the faceplate off.
Some OEMs (HP, *cough*) give you neutered windows reinstall cds that won't let you load extra drivers, and the windows serial number you get won't work with a generic windows install cd.
You're often stuck with low-quality components... cheap/slow/loud cdrom drives, cheap/small hard disks, no-name ram. OEMs cut corners wherever they think they can get away with it.
If you want a new computer for cheap, and you're willing to sell your soul and your sanity to save money, and you never intend to upgrade components other than ram and expansion cards, that's when OEMs make sense. Otherwise, do yourself a favor and get a real chassis and name-brand ram, a decent-sized disk (pair), a decent motherboard, and a decent dvd burner. I recommend newegg.com.
RAID 1 is mirroring, and isn't a bad idea given how cheap harddrives are now, but I still wouldn't recommend it. All it's going to do is a) cost you more money, and b) reinforce bad computing habits. Back up your data!
Backups do not address the same problem. Backups are not real-time, and if you have to resort to a backup, you've lost between a day and a week's worth of work. If you use some imaging program (rather than just selectively backing up your data files) and dump the images to another drive, then you might as well be using that drive for raid-1. Backups are for if your OS becomes unbootable, or if you mistakenly delete files, or someone steals the computer, or if you spill coffee on it.
How on earth does raid-1 reinforce bad computing habits? Very few people do backups anyway, and those who do are disciplined enough that they won't stop just because they have raid, at least not if they understand the dual purposes of raid and backups.
Yes, two drives are more expensive than one. You get something for that money. It's a hedge against having to screw around with backups and/or reinstalling if a single drive fails.
OTOH, most motherboards that support "raid" don't really. They come with drivers that make windows think the disks are raid. If the drivers get screwed up, or you can't get them to load when you install windows, you can't use raid.
AMD vs Intel... core 2 duo is somewhat faster than the top Athlon X2s for gaming and some other uses, but I'm sold on AMD's architecture and their roadmap. Unless you're building a dedicated gaming machine and need every ounce of computing power you can get, I'd recommend AMD. AMD is also much more power-efficient even than core 2.
One thing to look for on motherboards is an eSATA (external SATA) port. It's not a huge deal, since you can get an adapter that fits in an expansion slot and converts one of your motherboard's SATA ports to eSATA, but getting a motherboard with an eSATA port will save you the trouble. If you think you might use an external drive, that is. Sure, you can use USB2, but they're slower. Most drives can't keep up with SATA in raw read speed, but they can in short bursts if the data's already in the drive's cache.
