NUnit is good, but I think something that is more focused on Acceptance Testing or Functional Testing would be better suited.
TestComplete and to a possibly lesser extent,
WebKing seem to focus on these more.
In addition, I might be able to use the
MS ACT for this.
This guy thinks that
FIT(
.net version?) and the
FitNesse framework are the ways to go.
Microsoft has it's own
Testing Patterns and Practices section.
If all else fails... here are
tons of
links about functional testing.