Uh. When I think of RAID, think of redundancy and generally a loss of space. If you're that paranoid about losing data, then I suppose RAID is for you. But if you want space, then you might as well just go for a large drive. No, you probably won't be able to back it up, but you'll have more space. I suppose if you want performance, thought, you can always get a Raptor, or a 10,000 RPM drive to speed things up. Or one of those new fangled SATA 6's that I'm starting to hear about.
In this case, RAID 1 would be the exact opposite. You get the true amount of space from both drives and they run really, really fast. Downside - if you crash just once, all your data is toast.
So, no, I do not agree that you should run two large drives in RAID 1, especially if it's data you need to keep. Either get a raid controller that can do 5 or stick with single large drives.
2x750 should be plenty. Unless you're working audio/video where you need a huge scratchdisk, then even that may be iffy.