GPL Question

[Ryan]

I'm not sure this is the place to be asking this, so please redirect me
if necessary.

I'm working on a payment-routing server (http://ripple.sf.net/), and I'd
like to release it under a license that obliges those who build services
on it to also release the source for those services.  Applications built
on top of would essentially be accounting systems that interact with the
server over a socket (likely using HTTP), but that interaction is quite
complex, requiring several different types of callbacks to the
accounting system.  

Would the Affero GPL work for me?  Would an application on top of my
server be considered a "single system" with that server for the purposes
of the GPL?  

What if there was, or could theoretically be, another server like mine
that uses the same API, so the application could plug in a different
server?  Is this important, or is the fact that someone *is* using my
server tightly integrated with their application the relevant fact?

A similar example to consider might be if I was releasing a database
server under the AGPL.  If someone used it in their web app, would they
be obliged to release their app source under the [A]GPL as well?  Would
it matter if my db server or its interface was generic or
unique/specialized?

Thanks,
Ryan