No you can't, PSN cards are country and currency locked. You cannot redeem German PSN topup cards on a UK PSN account.
Many stores that sell video games also carry PlayStation Network cards
No, only PSN cards with dollar amounts on, can be redeemed on the US PSN.
16
PlayStation Network Cards for PlayStation Store purchases are currently available in $20 and $50 denominations. There are also $10 PSN cards
Maybe
yes they if you run out of money
PSN only accept Credit Card or Debit Card that belongs to the region in which the PSN account was created. If you have a US PSN account then you can use only US credit or debit card.This is because PSN does some background check called Address verification Check which is basically Sony checking with your card issuing bank about the address, if the address is from a different region then you will not be able to use that card for that account.Other alternative is to use PSN cards. If you are from a different region and you use a different region PSN account then the best option is to buy a PSN card online.More detail on this can be found at : http://invisibleknight.blogspot.com/2013/04/psn-cards-playstation-network-cards-how.html
Six zeros I believe :) 1000000
They still sell $10 Playstation Network cards
only from a scam of course
Mine had 10.