Need a new server: recommendation in converting my systems

[mmx]

I'm adding 2x 750 seagates HD that I have.
I need to act SOLELY as a file sharing and backups to an Imac, dell xps laptop, and macbook pro (Win and os x)

Which one will be the best and most cost effective - energy consumtion.

Apple dual 2.0 G5
Dell dual core 1.8ghz
P4 3.2ghz

 

[ViRGE]

The G5 was rather hot, I can't imagine it would be the most power efficient option. And this should be in General Hardware, not Operating Systems.