Guys, BillA's latest test results for heatercores are in
this thread.
His testing indicates that the single pass heatercores (like the one BrianW provided pictures of) have much lower flow resistance than an equivalently sized two pass heater core.
With a low flow resistance waterblock, like a Swiftech, a single pass heatercore should result in much better flowrates. With a high flow resistance waterblock, like a White Water, the flow resistance of the heatercore is much less significant.