Higgy that garden is awesome! Like Tony said, I'm taking notes!
A design like that woul work perfect in my back yard.
Check out the water barrell designs on these sites:
A Spouse's Guide to Building the Perfect Rain Barrel System
Rain Barrels, Downspout Diverters and Filters, Rain Barrel Kits
I think you have plenty of room for at least two, if you build a raised platform. Personally I wouldn't buy a rain barrel already made (cost too much) you can build one pretty cheap.