Building your own PC

I was considering of buylding my pc. As ive seen a bunch of crap being offered by companys that think costumers dont know squat. I have no prior experiance building pcs and so on and so on. I know about the main parts but all of the other technical stuff like power cources and other bits and bobs relating to that would throw me off.

Or i was considering buying a crapy pc and upgrading it. Good or bad idea.

How hard is it to unstall a new procesor and would there be any problems if i jacked up a poor pc with a new intel core 2 procesor about 1-2 gb of ram and 160-250gb of hard drive and a new graphics card. Would there be any issues if i done that?

Any bad experiances from building an own pc and is there any good places to buy parts online.