Well, the 980M's power consumption is rated at just around 100W, so allow 200W for the graphics cards, another 50W for the CPU, leaving you 80W for the rest of the system - a rough and ready calculation but I would conclude that 330W should be adequate although I'd expect the power brick to get somewhat warm or even hot when you're putting the system under considerable load. Bearing in mind that you're almost never going to have both GPUs and the CPU under the maximum possible load at the same time outside of benchmarking/stress testing, I can't see that the lower rated adapter would cause a problem.
Look at it this way - if 330W was not sufficient for the laptop as configured, other resellers would simply not offer it as an option. The 660W option gives you headroom for overclocking and, probably more importantly, a better margin for the reseller
