It all boils down to
a) budget, and
2) your intended purpose of the machine.
As for best places to buy online, take your pick between ebuyer, dabs, lowestonweb.com, savastore.com, scan, etc... If you're in a rush to get your hands on the kit quickly then I can recommend
PC Nextday - I bought a mobo from them and it arrived the next day (like they promised it would) and was excellently packaged. It wasn't any more expensive than getting it from any of the more popular e-tailers despite getting it the following day
