I did a similar transition into a cath lab from ICU. Our role was similar, but we actually had a full shift where we rotated as the 'x-ray nurse' for the day. You will want to spend quality time getting your anatomy down really solid. Thinks look different when the camera moves to different angles. If you can borrow one of those model hearts, you can hold it up and look at it from different angles and get the idea of what you are seeing when the camera is moving to different angles.
A useful site and possibly organization for you to consider is the American Radiologic Nurse Association (ARNA).
http://www.arna.net/?l=home&w=1440 They have a newsletter (don't remember frequency) that has useful articles on all sorts of radiologic info.
Enjoy the transition! Don't be too hard on yourself, no one walks into any specialty area knowing everything they need to know.
Good luck!