I think right now is the WORST time to upgrade pre/pros to handle the digital data stream from Blu-ray or HD DVD. Until they offer full digital video and sound processing from HDMI inputs, you're buying obsolescence.
So the interim solution is to make use of your analog DB25 5.1 input on your Sunfire. Here's Wireworld's offering of the DB25-to-RCA cable SoGood51 was talking about: http://www.gspr.com/wireworld/rainbow.html
Straightwire, Cobalt Cable, and several others used to make them. Maybe a vendor still has one lying around. You might also contact Sunfire or Aragon (who also used the DB25 input) for a wiring diagram so you could "roll your own" if Wireworld no longer offers theirs.
So the interim solution is to make use of your analog DB25 5.1 input on your Sunfire. Here's Wireworld's offering of the DB25-to-RCA cable SoGood51 was talking about: http://www.gspr.com/wireworld/rainbow.html
Straightwire, Cobalt Cable, and several others used to make them. Maybe a vendor still has one lying around. You might also contact Sunfire or Aragon (who also used the DB25 input) for a wiring diagram so you could "roll your own" if Wireworld no longer offers theirs.