If you buy the nikon d3100 is their a SD Card with it?

And… How many GB?

No not normally, unless you buy the camera as part of a 'package' from a camera store which might include 'extras' such as cloths, bags, memory card or an extra battery.

How many GB depends on how many images you plan to shoot and what file format (less room on a card if you shoot in raw than jpg for example. I always have several cards, ranging from 2GB to 8GB.

The Nikon d3100 does not come with a starter memory card. With a 14 megapixel sensor, you'll want a decent size memory card; larger if you expect to shoot High Definition video. For example, the current cost of a 16GB memory card optimized for video hoovers around $100.

btw, the d3100 supports Secure Digital, SD High capacity and SDXC memory cards. SDXC cards are relatively new and will have storage capacities of more than 32 GB up to two TB.

Unless there's a package deal somewhere on the internet (from a reputable source) you will not get an SD card with the camera.

If you plan on doing mostly hi-def video, the best would be a 16gb class 10 card. If you really don't focus on taking lots of video, perhaps a 4gb or 8gb class 6 or 10 card