I need simple answers about building a computer!?

So recently i decided that i would start trying to upgrade an old 2005 computer, until i realized that it was obsolete, or that more notably the motherboard was. I bought a new fancy videocard that was formatted for agp (because that was the only thing that would fit in the motherboard) but then i realized that there were no windows 7 drivers for that old crappy motherboard. So if i wanted to buy a new motherboard with a greater potential for upgrades (one that may also accept that agp video card) would it be possible to recycle some old parts from that 2005 computer? It has a decent processor and what not, also i have a nice external hard drive. But im just trying to figure out if its cheaper to buy a new computer all together? or to build a new one with some new and old parts. I'm fairly new into the tech world and can use all the advice i can get!