Like so many things, it depends
-on how noisy the original is
-on how much edge detail is present in the subject
-on the intended viewing distance of the print
-on your prioritization on image quality vs quality of the image, i.e. is it of an important subject or event, etc.
At DPC Prints (here), we require a minimum resolution of 150dpi to sell to the public -- it's been adjudged that this produces sufficient quality (on the right printer) to be commercially acceptable. For your image, that gives a size of about 25.5" x 16.8" -- close enough to 20x30 that you should be able to either enlarge the image a bit to fit, or just expand the canvas up to the print size and plan on covering it (or not) with a mat (with or without frame). If you want to enlarge to fit rather than border, it probably makes little difference whether you resample up in Photoshop (or other editor), lower the resolution to change the print size without adding/changing pixels, or let the printer resize the image -- just make sure your print image has the same aspect ratio as the paper to avoid unwanted cropping.
Personally, I (almost) always border/caption my prints, both to avoid resizing/cropping problems, and so that they are suitable for display with or without (expensive) mounting.
I have printed this image 20" x 30" -- it came from a 2MP P&S camera! Yes, it is noisy and pixellated (blocky, jaggies) when you study it from six inches away, but at a normal viewing distance (for a poster) of two-three feet it looks fine ...
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