i would get the dp 2.0. i know the 2.5 would technically be faster but not worth the extra $500. plus the complexity of the system is up a notch from the 2.0 since it is liquid-cooled. this tells me the chips are running _extremely hot_ and liquid cooling is a stop gap to tweak some more peformance out of the system. heat kills processors/electronics faster than bush kills iraqis. so the long-term reliability would be my main concern. a fan is much simpler mechanically than a water-cooling system so it is a win in my book for the 2.0.
i would ugrade the ram first, then the display and video subsystem before i got the extra .5ghz per proc. much better use of your money. seriously why bother getting a high-end mac if you can't couple it with at least a 23" cinema display? (yeah i know the 20" are respectable too)
but put at least 2gb of ram in that dp 2.0 and watch the performance ratchet up. it really depends though on the "sample" of applications you will be running. you know at any given moment what will you be using your computer for? games, video stuff, cad, checking e-mail, posting to macrumors? that is the most important question to ask first, what do i need this for and how much horsepower will i really need at any given moment. otherwise i think you are burning money for no reason.
but i personally am waiting for the dual-core 3ghz G5 machines. i know they aren't coming out until january but hey i can wait...
jaromski