I would say the biggest problem with an ATX case is size. As for the components, an ATX mobo and "normal" processor will draw more power, which means you have to power it somehow without draining your battery and potentially deal with more heat. For PCI components, I don't think you'll have much trouble with vibrations. My mini-ITX board has a PCI slot, some people use it for a better sound card. For the hard drive, supposedly laptop harddrives have better shock protection. But, several members on these forums have been using 3.5" drives without trouble for years.