Actually, WHS no longer copies all the files you add to it onto the system (main) drive. This was the way that it originally worked, but it's been changed now (I forget which Power Pack release, but I want to say 3) so that the files are copied to their final destination as you add then.
That being said, for the cost difference, I think it's still good to have a decent 7200rpm drive as the system drive in WHS followed by gobs of energy efficient storage.
My personal setup is:
- WHS running in a VMware Server 2.x VM
- 1024Mb ram, 250G system drive
- 5.5TB of WD 'Green' storage drives
I started out with only 1.5TB of shared storage and simply added the other drives to the shared pool as I needed. (That's one of the things I like best about WHS.. storage just works without need to reconfigure everything to expose newer drives...)
Its served me well and I've been using this configuration for around two years now. When Vail comes out (the next version of WHS, based on Server 2k8), I'll probably switch over to Hyper-V as my hosting environment.