I'm considering a Eurocom X8 with 980M SLI and the sales rep I'm talking to says I'll need a 660W AC adapter instead of a 330W adapter for 980M SLI. I understand that SLI consumes extra power but both the MSI GT80 and the Sager NP9377 with 980M SLI from xoticpc.com come with 330W AC adapters, without an option for a 660W AC adapter. Is there any reason why? Should I SPEND the extra $245 for the 660W AC adapter or will the 330W AC adapter be sufficient for 980M SLI?

I'm considering the Eurocom X8 configured with

120Hz display
CPU: i7-4710MQ OR i7-4810MQ
GPU: GTX 980M SLI
16 GB RAM
SSD: Micron/Crucial M500 480GB
HDD: 1TB 7200rpm

I've never overclocked before and I don't plan on it but I want to make use of my 980M SLI so should I spend the extra $245 for the 660W AC Adapter? Thanks.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 Sounds like the market people are going for the sales curve.
Going 600 watts realty sounds like overkill. Wit a little effort the 300 watt supply could have been make to handle 400 watts for short term heavy loads.
But ignore me. That is just a reaction.