Uh hoh... you need front buffer, back buffer and z-buffer for starters. Geometry (vertices/polygons) also take VRAM so 1 meg would definately not be enough... also different color depths take different amount of memory.
16-bit:
(1024[w] * 768[h] * 2[color bytes-per-pixel]) * 3[screen,back,z] = 4.72 megs
32-bit:
(1024[w] * 768[h] * 4[color bytes-per-pixel]) * 3[screen,back,z] = 9.44 megs
Check this thread for more spesific formulas:
http://www.blitzbasic.co.nz/Community/posts.php?topic=24157
16-bit:
(1024[w] * 768[h] * 2[color bytes-per-pixel]) * 3[screen,back,z] = 4.72 megs
32-bit:
(1024[w] * 768[h] * 4[color bytes-per-pixel]) * 3[screen,back,z] = 9.44 megs
Check this thread for more spesific formulas:
http://www.blitzbasic.co.nz/Community/posts.php?topic=24157