I will give you what my highest hopes are for temps ( which somebody will
point out that I have forgot something and the temps can't possibly
get that low). Water on the coldside at 0 deg C. CPU temp of 10 to 15 deg.
This is at an ambient temperature of 20 deg C.

I will do some more research and calculating to give you a more definite
estimate.
According to Swiftech's site, if I manage to get the coolant on the coldside
to be 0 deg C and I use an Apogee GTX ( currently the best block I own )
on my QX6700 and the load is 300 watts, the CPU temp will be around
30 deg C. Not what I am hoping for.

But, the cpu may get to higher speeds at lower voltage with lower temps.
So, I might get the QX6700 a fair amount over 4GHz without approaching
300 watts.

Still, I'll need a block that is better than .1 C/w to get below ambient
at highest loads.