640x400 @ 70 Hz is traditionally the video mode used for booting most VGA-compatible x86 personal computers which show a graphical boot screen (text-mode boot uses 720x400 @ 70 Hz).
320x200 @ 70 Hz is the most common mode for VGA-era PC games, using exactly the same timings as the 640x400 mode, but halving the pixel rate (and, in 256 colour mode, doubling the bit-depth of each pixel) and displaying each line of pixels twice.
At this point another difference between CGA/EGA and VGA has to be mentioned: Double scan in low resolution modes. VGA monitors never really display just 200 lines, instead they double the resolution to 400 lines by displaying each line twice, which looks quite a bit different than on a real CGA/EGA monitor.
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