This thread will give you a general idea of which resistance.
http://www.e-cigarette-forum.com/fo...ur-battery-temperature-preferences-excel.html
I bought a SmokTech EMDCC LR (1.7ohm) and I don't think I'll be using it much any more on my eGo-T 650mAh, my battery isn't powerful enough, just kind of bought it on a knee-jerk reaction. The dual coil lowers the wattage to the absolute low end.
With a 1.7 ohm carto, at the 3.4v that eGos output, you get above 7.7 watts, which is quite a bit. Going any lower with ohms I think could be bad for some of the components.
Currently I'm using an Askton Hughes MK-I Carto that is the standard 2.8ohm, and its giving me great vapor production and throat hit, but it seems to also be on the lower side of 4Watts.
From the chart above, looks like around the 2 ohm area is desirable.