There is no doubt that a external raid 0 is sexy and cool. It also makes it more expensive.
As you work on your Mac, you will see it is so easy to install and remove stuff. If you follow
my article on sata raid 0, I think you would be extremely happy with it. I know that I love it.
My next hardware addition includes a Raptor 10k 74GB main drive off of a second sata card.
That is going to be sweet. Now if you can afford an external Raid. Why not choose this setup:

Western Digital Raptor 10,000 rpm sata main drive 74 GB (I have seen it go for $155)
Firmtek pci sata card
2 x Western Digital Raptor 10,000 rpm sata main drives for a 148 GB Raid 0

Now that is some serious drive power. Even better than SCSI, some may disagree with that,
but try living with SCSI drives, they are a pain.

Good Luck
-maestro