Considering that it is easier for you to deploy the first version for 240 Dpi you can deploy 240 first than 320 (obviously if this does not require too much effort from your side).
Maybe you can release a package of image for 240 and another for 320 so each of use can just change them according to the device resolution.
I am not a graphic guy so I apologize if what I wrote is not applicable to your theme, take just as an attempt to help :)
Current Device: Samsung Galaxy Note
Current Rom: Paranoid Android 0.3
Current Kernel: Embended