The x-ray is done at 51 days counting from the first tie. It shows how many, how big, and in what positions. It should not be done earlier than this because it can interfere with fetal development.
An ultrasound is done from Day 21 to 28 counting from the first tie. They place a wand on her tummy and it projects an image onto a screen. It's not very clear. You never know if you are seeing the same baby each time or a different one since you only get a small window as the wand is moved around. So you can't tell how many or birth size as accurately as with an x-ray. One advantage is that you can see the heartbeats which you can't in an x-ray.
If you have the money, do both an ultrasound and a x-ray, but if you are only going to do one - wait for the x-ray.
I know that once a dog has a false pregnancy she is more likely to have another. Only about 10% of dogs need hormone therapy after a false pregnancy so your vet is right in saying that.
Of course, I don't know your dog, but she actually sounds like she will be a good mother. Don't worry about her playing with her toy that she was treating like a baby - she'll know the difference. The only thing I would worry about is her being so small so the birthing is more traumatic or if the birth requires a C-section they sometimes don't seem to understand that the babies came from them.
You really need to do a lot of studying on breeding and whelping - there is tons of info. on the web. Here's another good site - I suggest you read every section and then go to the next site and do the same.
http://www.homestead.com/malteseluv/WhelpingInfo.html