Your max res and fps are probably only going to be determined through experimentation. Level of detail, number of objects updating per frame, complexity of game logic interactions and your own coding efficiency are all going to have an impact on performance.
Ezzaral
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Since I don't know anything about the details of your code, graphics detail, etc. I couldn't really say. Just start with the 900x600 if you like and see how it turns out :) Once you have the rest of the code finished you'll be in a position to tune and tweak.
Ezzaral
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Well, one thing as far as your animation loop goes, you generally want to keep track of the time between draw() returns and use that to calc the next time to draw, instead of using a fixed 1/12 second loop. The reason is that updates may different slightly (or even greatly in some cases) and you will get jumpy refreshes if you don't take that into account. There are a lot of example implementation of this out there if you need a reference. Just Google on "Java animation loop" (I don't have any examples here or I would post for you).
Ezzaral
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