If it's RTGI and it makes a major difference, like Metro, then I'd sacrifice resolution down to like 1080p, I guess. Especially if that was with good overall IQ.
I'm not willing to go down to 30fps for it though. 40fps at the absolute minimum, and even then it would be a tough choice.
For the kind of RT we'll probably *actually* get - ie low quality reflections etc - then I'm not willing to make any (appreciable) trade off at all. When I can use it without seeing a difference, as with Ratchet, then I will, but for me it adds almost nothing to the graphical quality.