If your going to go with such a cheap build, I'd get 4 gigs of RAM (or maybe 6), not 8. Unless you're using a built in GPU, which I wouldn't recommend.
Don't let people discourage you from doing this saying you can't get anything good for $400, it's not true.
I built this for $407:
AMD FX-4130
Radeon HD 7770
4 GBs of RAM
320 GB 7200 RPM HDD
Asus M5A78 LX Plus Motherboard
Corsair CX-430 PSU.
Cougar Spike (this is the case)
All of this can, on some servers, run Blockland at max settings.
Do you live near a Microcenter? Because that's how I got my motherboard for $5.