Most of the A500 HD interfaces out there also have the ability to add more RAM to the system, this is nice
Some are combination interfaces in that they also have a CPU excellerator as well like the GVP A530 Turbo units plus RAM expansion and the HD. These are the best if you have the money to budget this type of expansion. Otherwise there are a number of cheaper solutions with just the HD and/or RAM expansion combo.
Most of these units are very easy to connect, you remover the cover on the left hand side of the A500 to expose the expansion connector then slide the HD unit into the slot and there you have it. Also most of these expansion units come with their own power pack/supply that needs to be connected to the back of the HD expansion unit as well