The good news is that if you follow the best practices in graphical design your application will scale nicely for most of the device versions. The bad news is that you will need to purchase several devices. At this time I would recommend at least an iPad2 since it can run iPhone apps. For Android, I would recommend at least one device with 800x480 pixels screen, but also 1024x600 and 1200x800 tablets. Please read a section on screen resolutions about details.
The most obvious solution for most remaining, missing devices in your personal arsenal are emulators, that is of course if you are not a game developer, or otherwise using acceleration-meter.