never ever bother with DPI in digital renderings :)
it's...complicated but just don't bother with it, it does't really mean or do anything useful in practicla terms in the digital realm.
leave it at 72 DPI.

there's a thread about this last week or two.
basically, all that matters is PIXEL size. if you want a high quality, large render, you need a lot of pixels.

the rule of thumb is you want 300 pixels, for every inch of print IF you want high quality print.
And you can always enlarge an image up to that  size, if the original one is large enough, and you have good enlarging software.

I render at 1680 x 1050 pixels most of the time (for my own desktops hehe) that's sufficent, when enalrged in Photoshop for 30 by 20 inch prints. There is some loss of quality but not much that ordinary folks will notice.

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