I personally don't think there's much you can do to prep for the WISC or WJ other than get a good night's sleep and let your child know something about where they are going, that they'll be talking to a person who will have questions for them, that they'll probably get to answer some fun questions, and not to worry if they don't know an answer.
However - I would also reassure you about one thing - not all HG/EG/PG kids start reading early or verbalize amazing thoughts at a young age etc. Your description of your dd is very similar to my EG ds at the same age:
She does do many amazing things. Her drawings are like those of a 2nd grader. However, she doesn't know ANY sight words. She can do a 100 piece puzzle or more, but misses some of the letters of the alphabet.
Try not to worry too much about the testing - I know that's easier said than done, but really and truly, while reading at 2 is a sign that a child is most likely gifted, *not* reading at two, and not knowing all your alphabet at 4... doesn't mean a chid is *not* gifted. If anything, it just means they are potentially less likely recognized as being gifted because our society has this expectation that "gifted" means "early reading, little professor" etc.
Best wishes,
polarbear
