Actually, in the framebuffer, alpha has no meaning. The remaining 8 bits are not part of the pixel display properties at all. It is there for address alignment so that all pixels in the framebuffer are dword aligned. This improves the performance of pixel read-write operations.
2nd, 32-bpp modes are available (and to the best of my knowledge work on the tnt2 vanta cards as well) on XFree86. If your configuration tool does not allow you to select 32-bpp, then you might need to edit your XF86Config file by hand (sorry). All you have to do is go to Section "SCREEN" -> Subsection "Display" and either make a new Display subsection that has Depth value of 32, or (if you are adventurous) change the value that is there to 32. You will also need to either change the DefaultColorDepth value to 32 or start your X server with startx -- -depth 32.