I would consider no raid with some sort of online data protection (such as Carbonite). This will maximize disk (750MB, say 600MB for data). Perhaps you split the drives into 3 250MB volumes and put data on two of them and the OS on the third.
Stefan's suggestion (RAID1 2 disks,l have a hot spare) is the safest, but only nets 250MB total disk. If you can make do with that little space, I would also recommend it.
Your second idea (1 drive unprotected for OS, RAID1 2 drives) nets 500MB, but only 250MB for data. I don't have a huge problem with leaving the OS unprotected as long as you backup your configurations to the protected drive, but I don't see it buying you much over 2 x RAID5 plus a hot spare.
Your first idea (RAID5 3 disks) is a compromise. You get more data space (400MB), but most experts advise against installing the OS on a RAID5 volume. If you do this, I strongly suggest purchasing a spare and having it handy so that you can swap quickly.
Finally, what I would really do if possible is get 2 small, fast drives, mirror them for the OS, then RAID5 the 3 500MB drives for the data, something like this answer to a similar question.