There are some seriously cute machines out there that I swear were made with little girls in mind. I agree about getting a Brother from Walmart. Their prices are very reasonable. They have a pretty decent selection in the stores but also online - you can do a ship to store or to your home. I'd avoid the ones that are the Sew Mini's... you would want something for her to grow in to and those small ones are too dinky to do much with.
She'll love the Xmas present - and if you get it in time to make her a nice dust cover to suit her personality and likes I bet she'll be absolutely thrilled... you could give her the dust cover in one box and when she looks all confused over what it is you could then give her the "big" present....