If you have any specific questions about a certain part feel free to ask, you will be amazed how much information is available from the forum members, and when you get around to deciding on some parts post on this forum again, we will help you find good deals and potentially better parts. Always feel free to ask for our reasons, you cant learn if you dont ask.
There are a bunch of PSU calculators available, but if you are interested in learning about PSUs, jonnyguru does PSU reviews and i have learned quite a bit about what makes a PSU good or bad from those.
Agreed, while its not the most legitimate source, it contains a lot of technical information written in a way that it is fairly easy to understand.
Just remember you cant compare across manufactures. nVidia uses a few slower SP's that get more done per clock cycle, AMD uses a bunch of small fast ones that get less done per clock, which is why if you compare the specs of the 4870 and the GTX 260 you would never guess that they performed similiarly. To compare across manufacturers check the charts section on this site, it is fairly up to date. You can also learn alot by reading through the reviews on this site, some of it will be over your head for a bit but you will learn stuff quickly.
most of the external stuff outside the case im recycling from my current computer (ie monitor keyboard mouse headphones)
im not really looking around to buy yet so i dont want people offering me buying lists and all that but my budget will probably be around under $1000 excluding ram and hdd (as i have recently gotten them for my current computer and will probably fit into the new rig