You're right about the StorageDevice automatically selecting if only one device is present. I also don't believe there is any way to force it to show the selection screen, as this is a dashboard feature that even licensed titles need to make use of. I don't see why you would need it though, you say "So that way the player knows that they are actually selecting a StorageDevice", but why not even give them the option and just assume they they'd rather save to their hard drive rather than the memory card they don't have plugged in?
As for the resolutions, 1280x720 is a good choice, but there's no reason why you couldn't support 1920x1080 (if you can find the performance) and perhaps a 4:3 resolution for those still using 640x480 or 1024x768 (on Xbox). If I were you I'd start with 1920x1080 until you decide you can't make the game run fast enough, then switch to 1280x720. Supporting different resolutions of the same aspect ratio is pretty straightforward, supporting multiple aspect ratios (for the UI) is considerably trickier.
I'm not sure about your DPI question though, perhaps someone else can help you with that one.