330W or 660W AC Adapter for 980M SLI

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? Thanks.

330W or 660W AC Adapter for 980M SLI