The voltage regulator on the 7800GT doesn't need to be watercooled. That's one of those things that is purely an extravagance. While it would not be a bad thing, it is unnecessary as long as you have decent airflow in your tower. Cooling the ram is easy enough with the microheatsinks.
While most cooling products that work with a 6800 series videocard will also work on a 7800 series videocard Zalman does not list the ZM-GWB2 as being compatible with any 7800 card. However, looking at an old 6800 and 7800 card (I have both here with me not being used) they do sport the same hole configuration around the GPU and my old NV6800 waterblock fits perfectly on my old 7800 card. That would lead me to believe that the waterblock you are talking about should work.