[mythtvnz] CF card as system drive

[Noel & Di]

Hi all
I want to use a CF card with an IDE adapter as a system drive & use my 
existing 2 x 250GB HDDs for the storage.
Currently run Mythbuntu 9.04 with 1 x 250GB drive as system / storage & 
1 x 250GB drive as dedicated storage (using storage groups).
Question. What size CF card for a frontend/backend machine using running 
Karmic, will a 4 or 8 Gb card be enough?
Thanks for your learned comments!

Noel Claydon