A bit of registration hassle, but I've had fast, free DEM downloads at MapMart:
http://www.mapmart.com/DEM/DEM.htmI used bogus registration info and have been downloading DEMs over 150 Kb/s with DSL. MapMart has a nice map that you can zom in on (if you zoom in far enough it shows you the quad name) and select the DEMs you want to download. Uncompress the tar.gz file then convert it with SDTS2DEM.exe and copy the resulting DEM to TopoFusion.