I bought a Dell Dimension 9200 with a Quad Core processor. It hasn't helped a lot but I'm reserving judgment on that until I get a second hard drive into the box. I'm suspecting that things are more I/O bound. Anyway, I didn't want to pay what Dell wanted for a DVD R/W drive instead of a DVD drive so I got the box with just a DVD drive. I tried buying a DVD R/W drive at Target but the connections are wrong for my box.
So I ordered a DVD R/W drive from NewEgg. I thought I was getting a deal because the drive had free shipping but as an OEM model, it didn't come with a cable ... and it turns out that SATA drives use individual cables (meaning they run directly from each drive to a port on the motherboard) instead of the IDE cables I'm used to. So I had to pop for a cable for $6.
Got the right side of the case off easily without tools but couldn't figure out how I was going to put screws into the left side of the drive - the left side is riveted to the rest of the case and is clearly not meant to come off. Got it and working with Ubuntu without too much trouble. Had to enable the drive in the BIOS but that was the only issue.
After browsing for info, I came across Removing and Installing Parts: Dell Dimension 9200 Service Manual
Turns out that you can remove the 3 screws screwed into the back of the blank faceplate for the 5 1/4" drive, put them into the drive you're installing and then slide it into the case. Very, very neat. Gotta get my camera hooked up to the box so I can show you. This was the nicest design that I've seen in a while.