As an owner of both a QS (QuickSilver) single 733, and a MDD (Mirror Drive Door) dual 867, I can tell you that the MDD's are more expensive (bought both on eBay) and noisier. I bought the MDD with the plan to have the fastest, latest, best looking Mac that could boot natively into OS9 as well as OSX. Turns out that GigaDesigns was the only maker of an upgrade CPU unit at the time, (dual 1.8Ghz) They were having problems because of Apple motherboard differences. When I called a few months ago they told me that they should have that upgrade available this summer. Then I asked them which Mac he would buy if he was in my situation and he said to get a first gen QS. It's quiet with good fan air flow. It will take a 1.8Ghz dual upgrade from 3 or 4 manufacturers, as will almost all the other G4 Macs. QS can have up to 1.5Gigs of ram while the MDD can have 2Gigs. The 2nd gen MDD has FW800 but will not boot into OS9. One advantage to the 2nd gen QS is that it has Big Drive support. (will use drives over 128GB) Of course, if you're going for speed you'll want to add a PCI card for SATA drives and a FW800/USB2.0 card as well. HTH STeve