The onboard PATA bus can do 133MHz/channel, so unless you really need more than 1.5TB of storage you should be set. You could even do set up a software RAID in Disk Utility for extra speed and/or security.
A hardware SATA RAID controller on the other hand would take some load off the CPU, compared to a software RAID - but if the box is intended to just be a server, then serving (and RAIDing) will be all it has to do, so CPU load shouldn't be an issue, really. And finding a bootable or flashable RAID card will no doubt be harder or more expensive than a straight SATA card.
NB if you have an ATAPI DVD/CD on the same PATA channel as an HD, I hear it slows the HD down. Someone will no doubt correct or clarify this statement if needed
Consider also a Firewire 800 card and Firewire-SATA bridges.
A hardware SATA RAID controller on the other hand would take some load off the CPU, compared to a software RAID - but if the box is intended to just be a server, then serving (and RAIDing) will be all it has to do, so CPU load shouldn't be an issue, really. And finding a bootable or flashable RAID card will no doubt be harder or more expensive than a straight SATA card.
NB if you have an ATAPI DVD/CD on the same PATA channel as an HD, I hear it slows the HD down. Someone will no doubt correct or clarify this statement if needed
Consider also a Firewire 800 card and Firewire-SATA bridges.