Honestly it depends on your usage. I started with a 74gb raptor as a boot, but ended up having to run 2 of them in raid 0 because I needed more room.
Always best to have a higher RPM drive for boot, but if your anticipating having a lot of things on it, then size might take precedence rather than speed, unless of course you have the extra money to get a few raptors and raid 0 them, or just go with a few 15k scsi's.
As far as your 2nd post on those 3 drives, the two 250gb's should be good (but I would personally go with 2 of the SATA versions) for storage depending on your need (they are great drives btw), but I would consider at least a 10k rpm drive as a boot (see above paragraph).