I'm using a transmission cooler with 2X 120mm fans. It's the perfect size (long rectangle) for the fans. The fittings were tapered, so I had to cut them off to achieve the correct OD (they had plenty of length), but otherwise, it was perfect out of the box.
Obviously, the alum. will have poor cooling characteristics compared with a copper core, but I very much like that the tranny cooler is only 1/2" thick (maybe less). It can lay where no heater core has gone before.
This system is the dually 1.4ghz Tualatins, and my water temp is only 4-5C above ambient when run at load all day, so the rad. is good enough for these low-heat cpus.
I would post some CPU temps, but this mobo has utterly worthless temp monitoring, and my compunurse reported 35C load temp as close to the core as I could get it. I'm not sure that is correct either, but seems reasonable.
I believe the tranny cooler was $40 from parts store. It came in a whole kit with tons of fittings and hose, so if you could find one for cheap without all the stuff, one might consider it. But at $40 you just as well get a pre-made copper one.
I just wanted to pop-in, and say that I'm using a tranny cooler.