each institution, so were are looking for the most common platforms. At the moment, the client is only looking at magnetic stripe cards, not Smart Cards. Web sites concerning standards would also be appreciated.
I think your question is a little too general.
The platforms are probably different for each company. (C++/C/Assembly/COBOL/etc).
Verisign does a lot of work with online payment transactions and security. They're usually pretty helpful if you're looking at something they deal with. You might have a look there.
RE: Specific standards--you might want to try some of the various standards organizations like ANSI or IEEE. I don't know what they've got specifically for financial secuirty; but, they're involved in many of the mainstream standards.