might be better to build from the ground up. I remember when my first water prebuilt pump went out it was a kingwin system (though I've seen 6 other brands with the exact same system) and also had 1/4" tubing. what i found was that after buying a pump from DD and some tubing, that my little kingwin's radiators and the stock tubing wasn't designed for that amount of pressure. (and hooking a 3/8" tubing pump into it wasn't fun either) leaks were emminent. luckily I test they system for 24 hours prior to install and was able to see the leaking before I installed it. So I tossed that aside and went for an all custom system. no leaks and a huge cooling performance gain. you'll find that while the prebuilts were good for a first run, they pale in comparison to the performance of the custom ones.
so I'd say shop around a 2x120 rad should be plenty (you'll find 3bys and 4bys hard to install) especially just for the cpu, if you're planning to add in the 4870 it will also be good, but a 3by would offer a bit better performance (but not huge) swiftec 2x120's are all pretty good, on DD stick to the xtreme and gtx, steer clear of pro and gts series. there's plenty of us around who can help, infact a great deal of tpu users are on water and I'm sure they'll be around to recommend blocks, pumps, rads, etc.
my recomendation though is start fresh, you won't regret it.