As a visual fx/design major I have to clarify this. I HATE misused terms. (That is the Virgo coming out in me.)
DPI = dots per inch (usually ONLY used in print, and does not define the dimensions of the final image. If you are talking about the space between pixels on a crt, that is "dot pitch")
resolution = number of pixels in a given axis (usually described as height x width etc)
colour/bit depth = the amount of colour/transparency information stored per pixel
megapixels = snazzy marketing term used by camera manufacturers that gives less information than it hides (what is the horizontal resolution? the vertical resolution? the colour depth? none of this info is present in the phrase "X MEGAPIXELS!")
3D software does not render according to "megapixels." You specify the resolution as x-lines by y-lines, and set the pixels for either square pixels (1.0) or some variant, depending on final output format (DV, for instance, does not use square pixels, instead they are set as 0.9, rectangular.)
So... what is your final format? TV? DVD? Blu-ray? 35mm film? Print? If print, what size is the final copy? If you just want a pretty image for your computer screen, I wouldn't render any bigger than the resolution you can display.
Wow...I sure do ramble...hopefully someone benefits from all this info!