For $400, you may be better off just buying a Dell (speed/memory-wise), but you can build a PC for cheap since you said you're looking for just a *good* PC and not an awesome one.
I've build some PCs and the one I use at home I built for $1000 (reused monitor from old PC). Of course, it has neons and a gaming case, but it's been 100% reliable and is FAST.
You can save some money (and time) by getting a barebones kit. It will have the case, mobo and CPU, plus you won't have to hassle with damaging the processor (easy to do) when installing it.
You add some cheap RAM, a lower-end video card from eBay or Newegg, a cheap sound card and modem and the drives and you've got a cheap PC. The ONLY thing I recommend is to upgrade the power supply that comes with the case. I'd never use a PS that comes with a case, it won't last. Antec is the best company for power supplies, get one of those and it'll last a long time.
Defintely go with an AMD, not an Intel. Bigger bang for the buck.
You can get good deals on barebones kits at http://www.pcsupersite.com
I've used them before and they have great prices and fast shipping.
Go to http://www.pcsupersite.com/pricelist.asp
for a pricelist. I highly recommend an ASUS mobo, I have one and love it.
You can definately get a good PC for your target price if you shop smart.