GreyAlien: ... You don't need any kind of alpha buffer in order to be able to DRAW with alphablending. Your PNG's will render find regardless of whether you have an alphabuffer.
Having an alpha buffer means that the screen/backbuffer ITSELF has an alpha channel. It doesn't refer to the alpha channel present in your sprites. You don't need an alpha channel in the backbuffer in order to render alpha blending because alphablending does not take into the account the backbuffer's alpha channel - it only uses alpha from the source sprite to determine the ratio of color blending.
Your 32-bit mode almost definitely will have an alpha channel in the backbuffer. Your 16-bit mode probably does not, or if it does it probably isn't 8-bit (ie 15-bit might have a 1-bit alpha), and on most systems it might only be 1-bit or 0-bit. It's very uncertain whether you will get alpha in a 16-bit mode. But that doesn't seem to matter for your system because you don't use the backbuffer's alpha buffer anyway.
You could support 24-bit modes, where the display/backbuffer does not support an alpha channel. Not having an alpha in these modes doesn't mean that your sprites wont have transparency. Alphablending still works regardless of having an alpha buffer in the display. Some people will have 24-bit screen modes but not 32-bit. On my ibook for example the desktop is 24-bit, no alpha, so if you opened a window it might not get an alpha channel.
It's up to you, of course.
In my system I'm supporting 32-bit, 24-bit, 16-bit and 15-bit mode, or windowed.
Having an alpha buffer means that the screen/backbuffer ITSELF has an alpha channel. It doesn't refer to the alpha channel present in your sprites. You don't need an alpha channel in the backbuffer in order to render alpha blending because alphablending does not take into the account the backbuffer's alpha channel - it only uses alpha from the source sprite to determine the ratio of color blending.
Your 32-bit mode almost definitely will have an alpha channel in the backbuffer. Your 16-bit mode probably does not, or if it does it probably isn't 8-bit (ie 15-bit might have a 1-bit alpha), and on most systems it might only be 1-bit or 0-bit. It's very uncertain whether you will get alpha in a 16-bit mode. But that doesn't seem to matter for your system because you don't use the backbuffer's alpha buffer anyway.
You could support 24-bit modes, where the display/backbuffer does not support an alpha channel. Not having an alpha in these modes doesn't mean that your sprites wont have transparency. Alphablending still works regardless of having an alpha buffer in the display. Some people will have 24-bit screen modes but not 32-bit. On my ibook for example the desktop is 24-bit, no alpha, so if you opened a window it might not get an alpha channel.
It's up to you, of course.
In my system I'm supporting 32-bit, 24-bit, 16-bit and 15-bit mode, or windowed.