Weight difference should be in the neighborhood of 1 to 2 pounsd heavier. Here is a link to the hubs dimensions info needed for determining spoke lengths.

http://www.shimano.com/publish/conte...Dimensions.pdf

This includes hub width info as well as spoke flange diameters information

BTW is it a steel or aluminum frame and what is the distance between the dropouts? this can have an effect on hub choice as aluminum frames cannot have the dropouts spread safely.

If the current hub has a coaster brake then presumably you will want the same on the replacement which may make finding a new one more difficult. Be sure of the features and OLD of whatever hub you buy.