AMD v. Intel, Nvidia v. ATI, I tend to tell people to pick whatever they're more comfortable with. There's a ton of spin and often the differences can be minute. Price differences on the other hand...
My experience with AMD and Intel has been thus:
Intel - Pricier, but comes with better tools and are probably the most reliable processors.
AMD - Cheaper for about the same power, intended for overclocking and modification...slightly less reliable.
I've had better luck with Nvidia than ATI. That rests largely on who gaming devs choose to work with when they develop games though. And I do know that the most recent gen of Nvidia top performers are power hungry monsters.
Either or will be fine. Don't let fanbois on either side of the fence convince you one is absolutely better than another. I suppose I choose Intel because I know a guy that works there.
PSU- You don't need anywhere near that much juice unless you plan to crossfire video cards, or are allotting power use for the next couple gens of hardware.
RAM: Corsair 1GB 240-Pin DDR2 (4 x)
Spend the money, get the best you can (which I guess is still DDR3). Corsair is a brand I trust, but memory is dirt cheap. There's no reason to not get the best you can afford unless your board doesn't support it.
HDD: Either 500 GB or 1 TB
Other than size, make sure you look at all the specs on a HDD. People tend to write them off because they just store data...but RAID and basic drive architecture have actually improved them in the last few years to where they can impact performance. So it's worth looking for a GOOD hard drive...as everyone loves lower read/write times.
And lastly, as some have said....
leave yourself room to work inside your case. Get a ruler out and actually visualize the dimensions of your case. There shouldn't be a case where stuff can't fit.....but if the case is too small, you may have clearances between parts of less than an inch....and that's no bueno.