I have a couple of ideas I think you should think about for that system.

First off get a third hard drive to backup on, something as big as the two 160's or bigger. make sure to seperate the hard drives as much as posible, and I like to use these on drive cooling fans as extra cooling. The raid bit makes the drives much faster but if either drive crashes you loose all the data on both drives, now the raid 10 bit with both mirroring and striping is good but in most cases a raid array for the programs and a third drive for data is a bit cheaper and some times more functional.

Put as many case fans in as you have spaces for, make sure to get a case with lots of fan locations and more drive bays the better too.

As too getting a premaid DEL, HP, whatever; I allways tell people a custom upgradeable system is better long term, as you can get new video cards, main boards, etc. and keep upgradeing rather than having to get a whole new system every time you need a faster/better machine; These package deals seem like a good deal till you try to upgrade and find you have to get special expencive parts from the manufacturer or get a new PC.