Motherboards are generally cheap. Unless you want some fancy schmanzy state-of-the-art 600$ thing, just because it has "that new feature that nothing yet really uses". There is a difference between good and bad motherboards though, and it doesn't really vary in price accordingly.
1) Google for some recent reviews on low-cost motherboards.
Stability: I've had 3 PCs since 1999. After LOTS of trouble (random crashes, freezes, reboots, overheating) with my PCs before that, I switched to AMD-processors instead of Intel. Honestly, on all of these 3 PCs, they've been so stable that I've been able to just let it stay on for almoast a year, and still do work on it/play fast-paced 3D shooters, etc. (With exception of when I had Windows ME installed xD)
2) Sturdyness, solid power, stability - Get an AMD processor.
3) RAM varies in price after speed/quality. Don't get the cheapest ones!
It sucks when you've put together the computer, and it won't boot. Just because some cheap-ass RAM-chip was destroyed when you recieved it. (You can even find lots of reviews on new RAM-chips)
4) Video-card.. Again, after having lots of dreadful issues with other video-cards, I went back to Geforce this time. Geforce 8800GT was cheap (one year ago!) and now you can probably get a much better model for the same price. NVidia seems to be very compatible, versatile, yet powerful, at a low cost.
Throw in a cheap DVD-burner. Buy acouple of SATA-harddrives. Now you have an awesome computer!
Mouse, keyboard and webcam aren't issues you have to think about when building the computer.