I only delved into assembling a PC myself when my current desktop commited suicide (the power supply fried my motherboard when it died). It is the hub of my whole network; I manage the family's music collection, photo printing and backups from it and I didn't want to buy a new machine and rebuild those configurations from scratch.
I purchased a new motherboard combo'd w/the processor and heat sink (installed) from Newegg for $78 (I kept the same specs of the original since I didn't want to mess up my copy of Windows XP). I also bought two 1GB ram chips for $60. The new Power supply was modular and 350W so I wouldn't have to worry about overtaxing it again ($40).
So for under $200 I pretty much made myself whole. Unfortunately, it always boots up saying there is no operating system. Once I hit CRTL/ALT/Delete the Windows screen appears and I'm good to go. I've been told its something in the bootup or the BIOS that needs to be configured, but that's more than I'm ready to fudge with.
I just need to bide my time till I can get my daughter a netbook for her graduation from elementary school and reclaim her desktop