The Inquirer posted an article on why Dell started using AMD processors.
So, how does HP compete? Easy, with AMD. If they are bidding against Dell, they lead off with Opterons and point out price/performance versus just price. AMD wins this one handily in the 4P space, and by large margins in 2P, but not as decisively. So, this means that Dell has to discount a lot, sometimes up to 50% to be competitive. (As a hint to all Dell buyers, get an HP Opteron bid and show it to your sales person even if you have no intention of buying HP, it will lower your bill).