I second the psu, if you don't buy one that can support extra internal/external hardware you'll be sorry later and they are real cheap. If you want to check what ram you can buy for your motherboard, try crucial.com their website will match it for you.
Have you considered buying one?, For £400 pounds here in UK you can buy one with a flat panel monitor, given that most companies build bulk it might be a cheaper option, though Id do some thorough checking on the component parts first. Personally I always build and scavenge from older systems, but this usually ends up costing me more.