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24" x 36" at 320 PPI should be more than enough. BUT if you have upsampled from a lower res image, it may look blurry.
By "unsampled" do you mean the cheak box at the bottom of image size options after going to Image -> Image Size?
I imported a picture at 8.9" x 6.8" (Aproximately) with the resolution of 180 PPI, would this make it look blury?
To get a true idea of image quality it is best to give height and width in pixels.
Yes. You can't add detail. When you upsample you will make the pixels very small, so won't see pixelation, but the image may look blurry. Detail happens at capture.