First decide on which CPU Intel or Amd then 32 bit or 64 bit then go with a quality motherboard like ASUS or any other name brand, then decide do you want the sound built into the board or do you want a stand alone sound card like creative labs.
Do you want to go with the new PCI video card or the AGP card; personally I'd go for PCI express. On the video card at the top end there isn't a big difference in performance that a human can see, my advice if you already like a particular brand stick with it get the most and the fastest memory on the card that you can afford between 256 and 512 DDR memory is good but DDR2 is a little better.
As for system memory get as much as you can afford 1meg to 2 Megs should do you I would go with two I Meg sticks. Cases go with whatever makes you happy but make sure to get at least a 500 hundred watt quality power supple.
The best thing if you buy stuff from a local shop they won't mind answering your questions and some will install the CPU for you at no charge. If you’re not sure about something look at on line reviews and talk to friends, just don't let anyone talk you into stuff you don't need. The best advice is read all you can on line check specs and reviews, and check on line resources like
http://www.tigerdirect.com/ and there many other online parts sources Anyway good luck and hope this helps a little.