It will depend on heatload (cpu/gpu), fans you use and their speed, setup, rads, etc, and will vary system to system.
And of course, the only temperature that rads control is the delta between air and water, so that is all you want to know for adding more rads. My systems is very similar to yours, in that I have an OCed i3770K, did have dual gpu 295, and now have a titan vs your dual 670.
Running prime 95 on my OCed i3770k at 4.7, 1.31v consumes ~100W, pumps, GPU titan at idle, guessing 50W, so say 200W max for sake of rounding including dual pumps wattage.
When I had just cpu at prime load with 360 rad with 1600 rpms fans.... Ambient was 25C, water temp 32C, cpu temp 78C.
The rad has nothing to do with delta cpu to water, ie 78-32 or 46C gradient, and with ambient at 25C, 71C would be lowest possible temp with infinite raddage. Another way of looking at it, the 7C delta air to water is all I have to improve with adding more rads, or more powerful fans.
Add a second 360 rad (assuming same fans and fan speed), now my delta will be near 3.5C (double rads surface area roughly halves the delta air to water). Double rads again to 4 x 360 rads, and now near 1.8C delta air to water. (in reality doesnt exactly work this way, radiators increase efficiency with higher delta air to water, so doubling rads, will not quite halve the delta air to water, but close)
cpu loaded only:
So 1 x 360 rad my cpu ~ 78C.
2 x 360 rads my cpu ~ 74.5C ( via loss in efficiency from lower delta air to water, would actually be few tenths higher C)
4 x 360 rads my cpu ~ 72.8C ( via loss in efficiency from lower delta air to water, would actually be few tenths higher C)
8 x 360 rads my cpu ~ 72C ( via loss in efficiency from lower delta air to water, would actually be few tenths higher C)
For cpu only loaded, what I considered sweet spot for me was 2 x 360 rads.
For gpu full load delta air to water with 1 x 360 rad was closer to 13c, adding second rad brought back to 7C. But no game ever comes close to fully loading gpu.
For running prime, folding, gaming, either 2 x 360 rads or 360 + 240 rad to me is the sweet spot for a cpu and 1-2 gpus. In real world use, gaming I never get above 5C delta air to water with 2 large rads, even though fans on 700-800 rpms.
The only way you can answer your own question is calculate your heat load, then go to martinsliquid lab
and see what rad/fans required to give you the delta you want. Then assume everytime double cooling, you nearly half the delta air to water.
Then if you want to improve cpu temps other ways (if already maxed rad effect) you can work on cpu to water delta by getting better water block, better tim, etc.