Ohms = Ω, it's just the resistance in a circuit. Your e-cig is basically a very simple circuit. If it's not Kicked, then your battery is putting out 3.7v (not really, it starts off at around 4.2v/4.0 and drifts downwards from there, but 3.7v is the overall baseline which big batteries maintain for a while). Lowering the resistance (the carto or atty), and keeping the voltage the same, will affect the two other variables in a circuit, which are watts & amps. If you know/control any 2 variables in a circuit, it's easy enough to figure out the other two.
Basically, low res = hotter & more smoke. It's much simpler then it sounds and I am not that awesome at explaining stuff at some crazy early hour, but you can download an Ohm's Law calculator for free, for just about any phone or computer, just google it, or here's one for the iPhone:
App Store - Ohm's Law
You know the volts (3.7), you know the ohms Ω (whatever resistance you are using. 1.8 in your example), this will tell you the watts you are vaping at, and the amps being used.
In your example, you are vaping at around 7.6 watts, and pulling about 2.0 amps.
I use mostly Resurrectors on 3.7v (the SmokTech, single coil, 1.7Ω cartos), and occasionally Boge 2.0Ω. Cartomizers work differently than clearomizers, and you can get decent vapor out of a 2.8Ω clearo on a 3.7v device (which would NOT be the case with for instance a Boge SR. A Boge 3.0Ω will s--k on a 3.7v device).
So, the super-complicated answer: if 1.8Ω is working out the best for you with whatever juice you're using, stick with that. I've never used a Vivi Nova, but from my understanding it comes with 1 of each head, so you can try all three and see what to re-up.
Shockwave, Stealth, Precise = are all the same thing (unless Kicked). They are purely mechanical mods, putting out 3.7v. The difference is mAh, which is milliamp hours, basically the only difference is, how long will the same battery keep going.
Bigger = longer battery life, putting out the same voltage (again, more or less, it drifts when not regulated).
tl;dr: if 1.8Ω head on Vivi Nova is making you happy at 3.7v, it's gonna work about the same on all your other mechanical mods. The bigger battery is not putting out more power, it just maintains the same power, longer.