How many gigabytes will I need?

I would say 15-20G (depending on how much you play)

It depends on how long you spend on the Internet. Minecraft and Skype for voice only calls should be fairly light, and 1 GB might be enough. Add video to Skype, and you might need 2 or 3 GB if you play on Minecraft and use Skype a lot.

For example, Netflix can be as low as 0.3 GB/hour on low resolution, 0.7 GB/hour for standard definition. In the days of Netmeeting, I had some low resolution video calls on a 56 kb/s dial-up modem. This worked out at up to 0.025 GB/hour or 40 hours for less than 1 GB. If you keep the video resolution low or don't use video, then 1 GB could be more than enough.

Skype uses approximatly 0.6 GB/hour.
Minecraft uses approximately 0.1 GB/hour.

Considering during that week you'll skype 5 hours and Minecraft 4 hours a day (28 hrs total), you'll need approximately 6 GB.

Depends how long your going to be there. Get the highest one you can buy. Tethering wastes a **** load of data. I spent 1gb of data for 15 mins of tethering. Probably because i was downloading updates for my mac but either way skype wastes a lot of data as well. I honestly can recommend you how much you should get cuz i don't know how long you will be gone and how much internet you use. For me id go with a 10gb-20gb data package for a month