When I was designing and selling, I saved all my files at 300dpi. It's the standard for the type of graphics design I was doing and I think it's the standard for most graphics illustration. I don't have time to design anymore but I still do digital scrapbooking in which all my 12x12 and 8.5x11 images are saved at 300dpi for printing purposes.
When I scan photos, I scan at 600dpi but once I open the photos in Photoshop to edit, I save it as 300dpi.
150dpi is pretty pixelated if you plan to print.