ive done shadow mapping (which self shadows) the performance hit on my card was about 300% for the whole scene. mainly cause it has no native support for it thus i had to do it software

I think a popular self shadowing technic is shadow volumes using vertex shaders.
This requires however at least 2 pass but is done per triangle/polygon.

Don''t play too much about this, shadowing can be cool and add to the scene but if you don''t plane to do a resident evil game (or horror, thrilling game) you might probably don''t want to do it anyway.
After all why would you like to see reality through a computer screen, just look around and you''ll see how boring it is.

let the shadows be simple circles on the floor

-* So many things to do, so little time to spend. *-