that's what you get buying from Dell and Gateway. They force you to buy expensive parts that you don't need, and parts that they've made partner agreements for. You know, if you made note of the parts they're offering you, it would be pretty easy to just pick up an identical set of parts from Newegg and build it. The only blank spot would be the motherboard, and most anything you get would be better than the Dell / Gateway motherboard. And building is fun!
My advice, get the P4 560. The 570 isn't that much faster than the 560, and 560 is only $87 more than the 550 so it's sitting at that nice corner of the price/performance curve. And, according to Newegg the 560 is $170 more expensive than the 550 on the street market, which means if you get the 550 you're basically paying extra to help Dell give deals on the 560 and 570. On Newegg 550 goes for $280, 560 for $450, 570 for $800. (My better advice, if you had flexibility, would be to get a different processor that is hundreds of $$ cheaper and will perform only 5% to 10% less. You only need a $200 processor to play games perfectly!)
btw I don't care much for Dell, if you couldn't tell... but they are decent if you don't want to build it yourself. AlienWare is better IMO, at least they counter the high price by giving you really good components (i.e. the 6800 GTO scheme) and not gimping you on upgradability or choice of parts.