Interesting...
with that formula the number of a 500,000 person poll comes out to
0.0014144271570014144271570014144272
with 1000 it is
0.032258064516129032258064516129032
make those into percentages and the 500,000 person poll has a less than 1 percentage point margin of error while the 1000 person poll has a little over 3 percentage points for the margin of error.
I say poll more people, get a more accurate poll. CNN definately has the resources to survey 500,000 people...easily.
Another question i have now that you have answered one...how do the poll'ees get picked?