I don't know a lot about raid either, but I can tell you if it's set up as a striped raid0 array it will show as 1 450gig drive, the risk is if either drive fails you lose it all, so it's not for everyone, for those they go for raid0 + 1 array, this gives them automatic duplication of their main drive, BUT it requires ALL the drives to be the same, the same as raid0 does thinking about it, so even if you wanted to you probably couldn't.
As to slave, Sata doesn't use jumpers, so my only guess is just plug it in and make sure your boot drive doesn't change in the bios, worst case it just wont start, but as each Sata drive has it's own cable\plug then it doesn't need to be told whether it's master or slave, just so long as the Sata drivers are installed it should be fine, I will be going the same way when I get more money and it's how I figured I'd plug mine in, see how that goes, should be OK I'd of thought