Dave the pump size depends on how big a waterfall you want.
Is the waterfall for scenery, aeration, water temp adjustment or all three?
If the waterfall is for part time use than a larger pump won't cost you so much in electricity.
A submersible pump might also be best for this application, and probably gonna need atleast 1/2 hp pump.
My submersible pump is supplying water from a stream, running 120ft up to the pond thru 1 1/4 inch pipe and putting out 12gpm, height above stream about 10ft., this pump draws 175 watts. Use the largest diameter pipe to reduce friction and the shorter the run also reduces friction loss.
A pump something like this.
http://www.homedepot.com/webapp/wcs/stor...ocStoreNum=1262
