a build up machine will not have same performance after six months is just marketing trick by sellers of these pc's in the market , in reality nothing like this exist . In pakistan pc's end up burning because people use cheap quality casing, wiring and power supply. Realiability of pc depends upon the components you used when building a pc.
you said there are other things attached as well, i can mention many other things, like twp more example modern games and application can easily use upto 8gb of ram while ur pc is stuck at 3gb cuz its 32 bit, and all modern graphics card used pci xpress 2.0 while u r stuck on old agp.
All the pc reviewers and geeks build system themselves they dont buy a branded..