Hi Globesy - welcome to the forum.
You can spend the $50 "credit" at the Connect Store (when you register, click the "I am in the USA" box) but you won't be able to buy any additional books there, because you need a credit card with a registered address in the US.
You can, however, use Connect "Gift Certificates" in the Connect Store, and "Leaping Gnome", in this forum, has been kind enough in the past to buy them for those of us not in the US (pay him for the certificate by PayPal).
There are, however, a number of other sources for books. If you enjoy the out-of-copyright classics, there are 15000+ books that you can download, free of charge, in Sony Reader format from
http://www.manybooks.net . If you enjoy fantasy and science fiction, Baen publish e-books in open formats (RTF) which you can use on the Reader.