Sorry to kinda derail this thread again, maybe we need to start a DPSS general thread.
Anyway, Alaskan, if you want 589nm out of this it's going to be a real challenge. Since it's going to be operated in a pulsed mode, you want an NLO crystal with a large phase-matching bandwidth. Also, you really want a folded cavity, since Nd:YAG does absorb visible light, especially in the 580-600nm region.
Nd:YAG also has two dominant transitions from the upper laser level to the 4I(13/2) manifold, 1319nm and 1338nm, as opposed to the 4F(3/2) -> 4I(11/2) transition for 1064nm, which is the only one (actually there are more (12?) but the 1064/1062 transitions are extremely dominant). 1319nm is only about 1/3 as prominent as 1064nm, so you will need to design a resonator that suppresses 1064nm (but doesn't eliminate it), and promotes 1319nm to a level where they are equal. This is incredibly hard to do, as 1064nm will quickly begin to dominate if given the opportunity. You also can't use cheaper broadband optics, because you need to completely suppress 1338nm (though this could also be done with an etalon, it would eliminate 1064nm).
There is a LOT more to factor into consideration as well, but if you're up to the challenge, I'm pretty sure everyone here would love to see a high powered 589nm.
Good call on the DPSS general thread.
Nailed it on your other points. Going to want the folded cavity for proper beam waist positioning too... Alaskan, RE: your other question regarding using lenses instead of curved mirrors, here's another good reason. If you're lasing on two lines/dealing with multiple wavelengths you don't need to worry about the change in refractive index for different wavelengths, seeing as mirrors are reflective and not refractive.