I agree with Aaron - at least 512 MB of memory, but 1 GB is better. Video memory will only be important for SUMA usage. Disk space depends entirely on the amount of data you will be dealing with locally (vs. a networked setup, like a RAID server). You will find that with the G4 processor, the speed of the programs goes directly with the speed of the CPU -- I usually like to buy 1 step below the fastest available -- that's my rule of thumb for the "sweet spot". But of course, now that I work for the govt, I can spend your tax dollars with wild abandon!!! Get me that 60" plasma display NOW!!!
For a desktop system, the more screen space the better -- when using AFNI, you tend to want to have a lot of windows open. But I've used AFNI on a 15" iMac with little more problem that the issue of window juggling -- which the new Expose' feature of OS X 10.3 makes easier.
Hope you are doing well, Lloyd -- long time no see --
bob cox