Display card memory size and screen resolution

I always have a question about memory size and screen resolution, in the old time it is simple, just multiply the resolution with color space will do, ie. 1024 x 768 times N [8bit:1, 16bit:2, 24bit:3], if I want 256 colors I only need 768K so that a 1M card will do the job. But after trying different distro I get confused, I got an old PC with integrated video using 1M shared memory, it uses Intel i810 chip, but I can not use the driver comes with the OS(3.9), so I use VESA instead. The best it can do is 800x600@16bit that's what I expected, coz 800x600x2 is just less than 1M. But strange thing is that, the same PC running ubuntu can do 1024x768 at 16 or 24bit, I don't know how to check it on Gnome but obviously it is more than 256 colors. Can someone tell me what's going on here ?