As pjz99 was getting at, your dpi doesn't mean much. What are the ACTUAL pixels of a sample image you are talking about?

You got an 800x600 image, or 2,000x3,000 or 4,096x4,096? You can change dpi's all day long, and those are STILL gonna be the same dimensions - so what are the image dimensions you are starting with? They might actually be fine for print dimensions - also we need to know HOW they are being printed - CMYK, inkjet, minilab digital printer, billboard, etc.?

Need more info - sorry for being vague and not actually giving you an answer.
-Lew ;-)