Be aware that with only one light, your shadows will typically look "dirty" because one light doesn't provide enough samples to do blurred shadows well.  You can get much nicer shadows if you're willing to accept longer render times, by adding more lights in a small area, simulating an area light:

http://www.renderosity.com/mod/forumpro/showthread.php?message_id=3173583&ebot_calc_page#message_3173583

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