Hiya JimN!

>First problem... on the DELL site I cannot even configure a system with a combination of ultra-66 and SCSI. Seems to me it should be a valid thing to do. Is there something preventing doing so??? Or might it be a dumb thing to do for some reason???

Should not be an issue. The two should work together.

>A second concern is the performance impact of adding "older" (slower) devices on the EIDE or SCSI 'channels'...
>I would buy a new SCSI (10,000 rpm, 80mbps) but I would like to put the 2 SCSI drives from my old system into the new box too. The question is, would this have any impact on performance of the new faster drive? That is, would this cause the adapter to operate at the lowest common denominator speed?

AFAIK, SCSI devices act mostly independently. The data sent through the bus is what counts, not the speed of the individual drives.

>As for the EIDE side, I would have the same concern regarding having a CD-ROM or DVD mixed with it. Any impact on speed of the faster HD?
>I also understand that EIDE typically comes with 2 separate 'sides' (I don't remember the technical term). If that is so, could one configure the HD on one 'side' and the CD/DVD on the other, and would that help performance of the HD?

"Sides"? Hmmm....you've lost me there. It's not uncommon to have an IDE controller handling both HD and CD/DVD but I don't think either device has an impact on the other.