Storage options for a repurposed server?

A few months ago, I got a really good deal on a dual Opteron server (APPRO 2128HS) and have it up and running with Windows Server 2008. It would be nice to use it as a fileserver, though it only has 2 36GB SCSI hard drives. The backplane has room for 6 more SCSI drives, though if I were to fill it with the cheapest refurbished SCSI drives I can find, it would cost twice as much as what I paid for the server, and won't fit 2/3 of what I have on my external drive I use for media.

Would it be possible to switch out the backplane and put in an interface that will work with SATA drives? Would this be a worthwhile move, or would I be undoing any performance gains by having server class hardware by creating a bottleneck?

I think you're right, and that way I could at least keep the OS and some apps running off the faster SCSI drives I do have. What would be the best way to connect it to the system though? I have vacant PCI-X ports, should I try to find a controller card on that?