Cases - Dell Designed
PSU - Some are ATX standard since early 2001 some are not. - Support.dell.com you can look it up yourself in the docs section
Processor - standard intel
Memory - standard micron, samsung, etc,etc
They had rambus because intel force them to use rambus
Expansion slots - standard pci/pcie/pcix/agp
Drives - standard drives, use their ugly green rails
They're pretty much all standard crap. If you buy a mini case, dont expect to put in more than 1 drive, thats given with all manufacture
Those higher end larger system can fit 3-4 drives.. but these are target for noobs so if you dont like it, you dont have it buy it.