Newegg.com, outpost.com, zipzoomfly.com are all pretty good places to find parts at good prices.
dealcatcher.com and slickdeals.net usually have great deals on computer parts.
How much you should spend depends on what you're looking for...a gaming computer, casual computer or what not.
If you're asking these kinds of questions, you might want to look into buying a pre-built PC...you might have a trouble assembling a PC unless you have help.
Pre-built PCs aren't too shabby nowadays, or you can visit sites like ibuypower.com and select your parts and they'll assemble it for you.
Answers to these questions will be totally different, depending on what you want to use your computer for. A $300 Dell is perfect for my grandma. Maybe it would be good for you too? I don't know. Start out by deciding what its going to be used for.