Running the 11b stuff on a separate channel from the 11g WILL prevent the 11b equipment from slowing down the 11g group, but be sure to choose well separated channels, such as 1, 6, 11, which are far enough apart in spectrum to eliminate interference issues.

Also make sure you are running current firmware/driver versions for the 11g, as there have been lots of refinements over the last number of months since the final 11g spec was set.