I can completely understand that you want the best for your child, but I think you might find it harder than you think to get in. The Catholic schools in our area are extremely good so therefore entry is very competitive. For instance, last year when I applied for my 1st child there were nearly 60 category 1 applications (that's children where 1 or both parents is a practising Catholic in the parish where the school is situated) for 30 places, and 15 of those places went to siblings. I will be applying for my 2nd child this coming year and I already know that there will be over 20 siblings in that year so that leaves less than 10 places for new families!! You will have to live pretty much next to the school to guarantee entry...
Priests are also pretty wise to the fact that some people start going to church just before the school application process begins and I've heard that some will refuse to sign your application form if they do not recognise you. Just to warn you!!
Lastly, please do think of those people who attend church each week because they want to and the fact that there is a fab Catholic school attached is just a bonus. If you do manage to get your child in, you'll quite possibly be taking away a place for a child whose parents not only have been attending church regularly, but really don't have any other option but to send their child to the s**t state school down the road. Sorry, a bit of Catholic guilt for you.