yes, the motherboard, etc, are important, and one of the main things that is important about them, the motherboard in particular, is their upgradeability... What I mean is this:

Most (if not all) PC manufacturers who make "pre-manufactured" systems (systems that are pre-boxed, sitting on a shelf in a retail store) use integrated motherboards, boards that have the video card, sound card, sometmes even the modem and RAM, built right into the board. This means that you could never pull out your video card and put in a new one, or sound card, etc. So you're buying a PC that you can't upgrade, and will have to replace in two years or less if you want to stay even remotely current. That's the first point. The second is that while your motherboard, processor and RAM are important, you could have the greatest board and processor in the world and it does nothing for you if the rest of your components are cheap knockoff product. Depending on what you want to do with the system, this may not be a big deal to you, but if you want to do anything with good sound, graphics, etc, you want good sound and graphics cards that can be easily upgraded to adapt to changing technology and your changing needs. The hard drive quality is also very important. System drives (hard drive, 3.5 floppy, cdrom, etc) all have moving parts, and so are more likely to have probles later. Some of the moving parts can become misaligned and damage the drive, especially if you started out with an "off-brand" or low quality drive. This can result not only in loss of data, but in expensive repairs (which is another good reason to have a good warranty).
So, while the motherboard and RAM, etc. are certainly important, getting a good board and processor and then skimping on the rest of your system does you more harm than good in the long run.

-Branwen-