I have two 150gb raptors on raid 0, and a 200gb cheapo WD drive (cant remember what model).
I use the raid array for working from and everything (windows, installed apps and games etc) and just use the slower 250gb drive for backups. It just holds copies of anything that is important on the raid array. If the raid array ever did die, I could rebuild it with everything from the single drive (and vice-versa).
Ok if my whole comp got nuked and all the drives got fryed I'd be screwed, but I'll take my chances for the added convenience of not messing with making DVD backups every few weeks.
Although my raid array can hold 100gb more than my backup drive, I've found that in actual use that the backup drive is plenty big enough, because of all the space wasted to by installed versions of things and other day-to-day crap I dont care about backing up.
One more thought: If you're more bothered than I am about backup security, you can get an empty external drive case with a usb interface for like $30 or something.
That way you can keep your third drive with all your backups on it in a closet or safe (i.e. not plugged in to your computer all the time). So there would be no chance of losing your backups even if your whole PC fried. you'd have to connect it every now and again to update your backups, and copying over USB would be slower than having the drive internal, but if safety is your main concern its very safe and still pretty convenient.