building is easy. just read the manuals and follow the instructions. it is very easy and will save you a ton of coin... Take a look at a 1200 dell system, You could build a better system with a 64 bit amd , dual harddrives, better sound and video card for 800....
Plus building the pc will help you with troubleshooting and doing tiny upgrades down the road.
And the satisfaction of building something that works.
You could take a few days pricing out stuff... Either start with the AMD cpu you want and then find the recommened motherboards , video cards or Pick a motherboard and work you way out that way.
AMD CPU's will save you a ton of coin over intel... The price of a video card
As for cases I recommend you spend a little money and get a good case where you can remove both sides to get at everything , and also get a good powersupply, and good fans for proper ventalation.
ncix would be a good place to start for pricing out a system.