Hi, I am a newbie to linux too, but I can help in a few ways.
Umm, the power supply, yes get the biggest you can find that is also quiet, most of the new ones are resonably so, but you will need it eventually, with all the extra hdds, cdroms, dvds etc.
As for the rest, well it all depends on what you are intending to do with it, I mean, just study and office stuff, or big resource hogs like database apps, or editing movies, or playing yew bewt games and so on. Those choices will help in making a decison. It also depends on the amount you want to spend. I know when I built mine it took a few months of reading etc before making my decison, but that is 18 months ago now, so it is way out of date
well, before I bought it, it was old, but there you go eh!