Well, I think building your own is a great experience. It can usually save you money but not always. Some research into PC components, parts and compatibility is necessary but when you are done you will be much more tech savvy and have a great degree of personal satisfaction. As well as a PC that was built to your specs with your exact needs and budget in mind.
IMHO the best place to shop for PC hardware is
http://www.newegg.com another good place is
http://www.tigerdirect.com TigerDirect also has Barebones Computer Kits which have most of the parts you need in a DIY kit minus the OS.
As far as using your old computers parts, if your PC was using a Pentium 4 or similarly aged CPU I would stay away from that for these reasons:
#1-Very few of those parts will be compatible.
#2-They are OLD and could die at any time. PC years are like Dog years on steroids. An 8yr old PC is a fossil!
#3-They aren't worth much. You could try to part it out on eBay. You won't get much but, you may find someone in need of parts for an old PC.
#4-They really, really, really, aren't worth much! To give you an example: when I first bought my Dell Dimension 4700 I bought 2-1GB sticks of DDR2 for $480, 3yrs ago I added 2 more for $47. Back then a 160GB hard drive was about $160. Now $165.99 will get you a 3TB hard drive (3,000GB). That's 18.75 times larger for almost the same price and the same goes for the rest of the hardware.
If you decide to build your own or are even just entertaining the idea read this thread
http://www.tomshardware.com/forum/261222-31-build-advice You don't have to know everything in the template but the more info you can offer, the better the quality of response will be. Don't be afraid to ask. After all none of us new anything either, until we did.