I stick with 300 PPI (Pixels per inch since DPI dots per inch isn't really accurate for CG) until I get to large size projects like posters at which time it gets a bit unwieldy. For example, say you're doing a cover for a hardback book. Basically, your print size is 6.125 by 9.25. At 300 PPI this gives you an actual image size of 1838 by 2775. What I tend to do is make templates for use in Photoshop of the sizes I tend to use most, for example hardback covers, CD covers, paperback covers, DVD box covers etc. That way I only have to do the math once as I'm very bad at math.

Talent is God's gift to you. Using it is your gift to God.