I am moving to valencia next month with my wife and 22 year old son. I originally thought I must obtain an address, then attend my local police station to have the application processed. I have now found an internet site that offers to process the application and obtain the visas before we go, for a fee of 630 euros. Does anyone know whether this is a viable and cost effective option?

where you from? and where you living now

NIE
dont waste you money and as you loacl Spanish Embassy and thay tell you the same as i going to get when you there go get it and i save you 630 euros and i am not try to rip you off , now that be a drink for me that you have to get to me

I would suggest you contact your local embassy.  A lot of it depends on your EU status.  If you are a EU citizen, the process is so much easier.  If not, definitely contact the embassy to see about the NIE before you get here.  I am new here to Spain myself, but I would suggest calling your closest Spanish Embassy as your best bet right now.