I have and use an eTrex Vista Hcx which is very similar to your Venture (i think). City Navigator is the Garmin software maps/program that can be used with the eTrex. It has route planning and the ability to load the routes from a computer to your unit. Its expensive though and has a poorly designed user interface, IMHO.

For free you can download the BaseCamp software from Garmin. This software allows you to load GPX files onto your unit. You can use the the Bikely online routing system to create the GPX files.