Has anybody integrated with Paymentech / Orbital ?

use Net::HTTP to send and receive your XML. You can use Builder or
XmlSimple to generate the xml if you need to (both libs are built in
to Rails) but I'm guessing the XML is pretty simple.

For reading the XML, again depending on the complexity of the
response, XmlSimple or Rexml should do the trick.

The hard part is worrying about timeouts and errors. They should have
some recommendations in their documentation regarding the best way to
handle those.