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It will be interesting to determine the smallest genome (in genes and in base pairs) that is required for different types of microorganisms.

According to this http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/G/GenomeSizes.html

Venter assumes that 381 protein encoding genes are required for Mycoplasma genitalium.

and that the smallest genome yet found in a free-living organism is Pelagibacter ubique with 1,308,759base pairs and 1,354 genes.

However, the tiniest genomes ever found belong to two types of bacteria that live inside insects. One of these types of bacteria, Carsonella ruddii, is so small that it could perhaps be considered an organelle within the cells of the bugs. But both microbe species face the threat of extinction because of their small genome size.

C. ruddii has the fewest genes of any cell known in the world - a mere 182, according to the new results.
http://www.newscientist.com/channel/life/genetics/dn10259-bacteria-boast-the-tiniest-genomes-to-date.html

I just found this http://genomicron.blogspot.com/2007/06/some-big-and-small-genomes.html informative web page.

and this http://www.cbs.dtu.dk/databases/DOGS/index.php for genome sizes