I want a new one. I have been using recycled p.o.s.'s for too long, but I don't have a lot of loot.
Should I build my own?
It depends on your needs. If you just need something for internet and word processing, just buy a system from Dell for < 400 bucks.
If you are looking for a more top of the line computer that you can play all the latest games on and use as a Digital Audio Workstation, then I'd build it if I were you.
What mobo should I aim for?
Gigabyte or Asus. Do NOT skimp on the mainboard, it's easy to do but it's a big mistake.
As far as everything else goes, RAM and storage space is cheap so buy lots of both. For a processor, I'm an AMD guy myself but it seems Intel has a step up on them now with the hyper-threading technology in their processors. Always buy a couple of speeds down from the fastest processor, that's where you start to see a good price/performance ratio. Stay away from Celerons/Durons. Buy a video card based on your gaming needs. If you aren't going to play games then don't go nuts, just buy a decent one. Buy a cheap sb live sound card unless you need something for transfers. Most other things you should be able to reuse from your old pc.