What's always annoyed me with
"brand-name" PC's is that the packaging ALWAYS
 deviates from the "norm" established by the DIY market. Consquently, your
 DELL, Gateway, or Compaq was never upgradable, because the CPU or hard disk
 wasn't supported by the BIOS, or some peripheral wouldn't fit in the system
 because of address conflicts. Simple things like the on-board video caused
 problems when they failed because one couldn't disable them, or, if one did,
 the now-broken native video interfered with the replacement. Even printer
 ports posed such problems. Many times the packaging itself was the problem.
 Have you ever tried to replace or upgrade the CDROM drive in an HP PC? 
It just depends on the model.
We bought several Dell Precision Workstations, 410s and 420s.
They use non-standard mobos and non-standard PSUs. Everything
else is standard. We also have Dimensions, which have similar
attributes w/r/t peripherals.
-dq