This article is from the comp.sys.ibm.pc.hardware.video Frequently Asked Questions, by Michael Scott with numerous contributions by others. (v1.0).
To determine the amount of video RAM you will need, use the method outlined in "How do I calculate how much VRAM/DRAM I need?"
There is another issue which is important to consider if the video card in question is a 64 bit card or is 32 bit but uses memory interleaving. Standard DRAM is addressed 32 bits at a time and typically one 1 MB bank is 32 bits wide and 256 kbytes deep, so if 2 1 MB banks are installed, each can be addressed separately. In practice, a 64 bit video controller can move 64 bits in or out of video RAM in each clock cycle, 32 bits to/from each bank. If only one bank of DRAM is installed (i.e. 1 MB) then the effective bandwidth of the card is halved. In a similar fashion, a 32 bit controller that supports memory interleaving like the ET4000W32i/p can move twice as much data per clock cycle if it has 2 MB of DRAM installed (see "How does memory interleaving work to increase the speed of a video card?" This means that 64 bit cards that have only 1 MB of video RAM will operate much more slowly that if a full 2 MB is installed. This may result in lower refresh rates and more sluggish performance.
 
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