Oh lucky you!!!
I don't have the 7700, I have the 6600 and based on that I would say test drive the following ...
Different thread weights in top and bottom. I've found it to be fussy with 60 wt in bobbin and 50 on top. Play, play, play with tension.
Accufeed foot, make sure you like it.
Play with some of the designer stitches including the letters.
Make sure you test piecing cotton as well as quilting it.
Also, I like piecing with the 1/4 accufeed foot. I use this foot and then re-position the needle for my "scant" 1/4 seam. Try piecing with it. I pieced tons of 1" strips and I will tell you that the foot is SOOO wide I couldn't hold my strip on the left of the needle to guide it through so I would guide it with pin heads I used to pin it. When I get near the end I have to use a stilletto for the last 1" or so to make sure it doesn't "migrate" to the left.
Also check out what attachements it comes with. Does it come with the single hole plate or just the regular plate? If you have to buy the single hole plate or any of the other attachments (like the 1/4" accufeed foot), then figure those costs into your total purchase.
Good luck - it's a great machine.