Mac G5 And Logic Need Help, advice on buying Options

[bertie mac]

Hello

I am currently using a mac g4 (440mhz) and cubase. I am looking to upgrade to a mac g5 and switching to logic.
I will be using plugins and audio exclusively to begin with (although I would like to add hardware modules to my set up in the future.)
My question is will I have to buy the top of the range G5 in order to get the best out of logic and it's plugins? will a lesser spec. be ok?
Also will I still need an external audio processor?
What features will I definately need to consider with the G5 and which features can I ignore - baring in mind that I will use the G5 only for music applications.
Any advice would be really appreciated! thanks in advance.

Confused Rob!