[CentOS] Infrastructure HELP!

Hi Everyone,

I started a social networking site and I am getting ready to go live  
next week. I have limited fund as I am not funded yet (although  
hopefully soon).

I have an extra Mac Pro (I know, how can I possibly call a Mac Pro  
dual 2.8 quad core with 16gb RAM extra). So Mac Pro quad Core, 16gb  
RAM, 4 x 1tb RAID level Seagate drives. I was going to load CentOS 5.4  
on it, web, mysql etc, etc. This is really the only box I have that  
would handle the site it it takes off and then I would need to add  
more hardware and most hosting to RackSpace or something.

My question is about initial setup. The 4 x 1TB drives. How to set  
this up for I have some protection?

RAID 0+1? (striped + mirrored) I would end up with 2TB useable space.

RAID 5? so what one is a hot spare? 3TB useable space?

What about striping the 4 1TB and mirroring that to a 4 x 1tb External  
drive enclosure?

I want to be sure the data is protected, but machine resources and  
money are limited.

Thoughts are appreciated!