In a way, determining the parts needed for a build is quite hard if you do not deal with these issues on a regular basis. Many times it is a matter of preferences which determine what hardware is used. What i am talking about here is the differences between Nvidia and ATI video cards, or Buying a Pioneer DVD drive instead of a Lite-On DVD, and such as that. Many times it is just personal preferences.
Just like i will never buy any products sold by PNY simply because many years ago they denied me 4 rebates simply because i bought too much hardware items from them. What i had done was buy some memory and a Nvidia graphics card six months previously. To me what difference does it make how much you buy, if in small quantitys, because they are the ones offering the rebates. Maybe their products are just fine, but i will never buy them.
OK, on to what you want. You did not say what you wanted the computer for so i will use generalities. Gaming computers need very good graphics cards like the Nvidia 8800 and plenty of memory in amounts of 3 to 4 gigs. For this you also need the faster( more expensive) processors and the better motherboards. I sell top of the line gaming computers for around $2000-$3500 ( there's faster and even more faster that is more expensive).
At the other end of the spectrum is the day to day computers that people want, but they want better than factory computers. My BETTER home computer include Creative SB Audigy sound cards, Nvidia 7900 or 8400 video cards, 2 gigs of memory, at least a 250 gig hard drive, Usually a slightly better motherboard ( i like Gigabyte, MSI, and Intel boards), the coreduo cpu's, and they average in cost at $700-$1000. I can build cheaper computers but i hate to do so. Better to buy a factory computer than buy a cheapie from me.
The best place to find out what you want to build is to read websites like Toms Hardware, and other sites like the Directron store.http://www.tomshardware.com/us/http://www.directron.com/byo.html
On building computers, heres some of my favorites.http://www.mysuperpc..._computer.shtmlhttp://computer.hows...-a-computer.htmhttp://www.build-you...-computers.com/http://techreport.com/articles.x/13671http://www.techpowerup.com/articleshttp://www.modders-inc.com/
AH, so many sites and so little time.
Google is also your friend. You can search for many topics just by typing in what you want to look for. I have done this for many years and plan on doing it for many more. I am always looking for more knowledge and others experiences. There is also this forum where many of us will tell you what may be better for your needs or may be better hardware then what you are planning on using.
I had so much fun building my own first computer after buying 4 or 5 factory computers, that now i try making some money from it. I don't make a lot but still like doing this so i will continue. If i was really smart and wanted to be into computers i would have learned programming. But i did not really like programming so i build them. I do need to say that when i was interested in learning how to build them, i went to a local computer auction where they were selling the old computer's at the honeywell offices. I bought a Pallet load of computers for $500 and the rest is histroy. Now these were old pentium 100 to 233 computers, and i think i got at least 10 of the 25 computers running. It was too bad that they all were guaranteed to run when i bought them. I guess i tried too many experments and fried a few of them. GREAT FUN THO!