MacPro Hardware for HD?

I know HES has not officially checked of on the MacPros, but it seems from reading the forums that many of you are using them.
I will be using Catalyst to serve our new DPI Lightning 40HD, and I want to get the best hardware that is available now. Hopefully the hardware will have some future as well.
So MacPro users, is there a difference between 2.0, 2.66 and 3 GHz processors in this application?
What video card are you using?
What Hard Drive configuration are you using?
How much RAM is enough? How much is just wasting money?
Thank you for your help.
Looking forward to joining the Catalyst community.
-Michael