One thick 360 rad with 80-90CFM of air going through it will keep a 4.0Ghz 920 and a Stock 480 at decent temps.
only in benching would temps go a bit high and even then you only bench either the CPU or GPU, rarely both.
I find that only in metro after about 3 hours gaming does my slim 360 with 60CFM fans start to heat up the liquid by 3C per 30 mins.
Im sure with a nice 2X 240 or even 1X240 + 1X120 you will be able to curb your CPU + GPU heat.
Note: I use a Cpu and Gpu block only, I dont cooler the vrms on the card or any of my motherboard with water.