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You definitely want to get the Swiftech APEX kit, it has everything you want. The best performing waterblock you can buy, paired with a highflow pump, and a dual heatercore which will be enough to handle the massive heat being put out by a dual core P4 at 4ghz.
Depending upon what else you have in the loop you would only see a marginal improvement by going from 1 to 2 dual 120mm radiators. One is plenty for the CPU, and probably good enough for CPU and a GPU. SLi might be pushing it a little bit.
I have to agree, I ran my APEX system on an A64 passive to to see how well it would do and it still only got up to 46c load running Prim95. And that was on the older high temp reporting bios.
The parts in that kit are about the best you're going to get for retail products on the market. That is unless you want to start using chillers, pelts and the like.
You might want to get a huge heater core later if the Swifty radiator can handle the power that you're putting out.