I have ask my self the same question, and I think if your not into OCing then the Toxic is great you just stick it in. But if your into OCing then from what I have gathered you can only up it like 10ghz. Its not like an asus or something where they have a tool for doing it. Yeah running cool is cool, but I would get the 5870 even the cheapest one, and OC your self and do your own cooling, which is what I am into I like being able to adjust myself and setup myself even with the cooling.
Your not going to be saving that much money even in a CFX2,3,4. You could get 2 5870's that would romp just as good as 3x 5850.