actually it is still handy as if it is used for part of the framebuffer then you would see a performance incrsses for that part, however you would need the base address for the sram to be someewhere in SDRAM so that the rest of the pixels arent mapped to a bit bucket in the sky.
basically it means you dont hammer the SDRAM all the ttime and with 16 bit at 6440x480, thats a large part of the mem bandwidth gone, intel even has a data sheet detailing its effects and how to calculate the bandwidth used
if i remeber correctly its 640 * 480 * 2 * 30 (hieght, width, Bytes per pixel, framerate) = 17MB/s
SDRAM = 400MB/s in 32bit mode or 200MB/s in 16 bit mode (cant remeber how the Z was wired),
dosent sound like much for displaying an image, but when we are talking about moives you have to factor in the write as well so 17MB/s becomes 37MB/s and with some moive formats you dont decode image 1 then 2 then 3 but 1 then 3 then 2 or some other wierd format (mpeg 4 with b frames bieng the worst) then you see that while the sustained read and wirte is the same, the peak burst speed is higher, basically you get stutering
then any type of buffering affects this and user spaace libries/programs might stage the data before copying it elswhere in memorey, so you see it adds up
