[LAU] Mobo/CPU advice for a DAW?

I'm trying to gather up a rough price quote for a friend for an Ardour Linux DAW station. His 2.3Ghz Core2Duo MacBook Pro running ProTools is starting to creak under the weight of tracks and plugins. I have not given up on trying to convert him to Linux and Ardour yet. If I can save some research time by starting from references from the list, that'd be a big help. The audio interface will most likely be an RME Hammerfall PCI, so shared-interrupt issues shouldn't be a big deal. I will probably either install a Firewire card (for external drives) or two, but the audio interface will not be on Firewire. I'm looking for an affordable system that can stay in service for many years, say 3-5 years. Any recommendations for what CPU and motherboard to get? I can find quiet cases and drives locally, that part's not too problematic. Thanks. -ken