When Windows wants to display something on your monitor, the communication between Windows and your tube is handled by a screen driver. A screen driver is an interpreter of sorts - it listens to what Windows wants to draw and expresses these requests in terms your display hardware can understand. When Windows wants to draw a line or the colour green or a word in baroque-looking text, it's up to your screen driver to see to it that Windows gets its way.
Actually, there's a catch to this. Your driver is only obliged to see to it that Windows gets its way within the limits of what it thinks your display hardware is capable of displaying. Being a bear of very little brain, a screen driver is easily confused in these matters, and most screen drivers spend their entire lives being lied to contemptibly, never knowing any better. More than a few of them have required therapy.
There are two primary characteristics of your display hardware which will affect what a Windows screen driver can show you, to wit, the dimensions of your screen and the "colour depth" it can reproduce. Screen dimensions are measured in pixels - it matters not a jot how many inches separate the edges of your monitor, and if you find a ruler and measure this distance you'll probably discover that the subspecies who sold you the thing rounded up quite a bit in any case. The pixel dimensions define how much stuff Windows can display on your screen at once.
The pixel dimensions of Windows screen drivers are typically one of the following:
The colour depth defines the number of distinct colours Windows can display at one time. Colour depth is often defined in bits. The actual number of colours can be worked out as 2 raised to the power of bits. As such, a display driver which can display four bits of colour can display 2 raised to the power of 4 colours, or sixteen in all. A display driver that can only display sixteen colours is pretty shameful, and probably requires reorientation and sensitivity training.
Common colour depths for Windows are:
Colour depths of four and eight bits use what is called "palette" colour, which means that all the windows visible on your monitor are constrained to work with the same palette of sixteen or 256 colours. What this means in practice is that the foremost window will probably be able to display colours which are more or less what they should be, and all the background windows will look like drunk stockbrokers who've just returned home after a night of playing "pin the tail on the paint spraying machine."
In fact, 256 colours are not sufficient to display photorealistic images as they are actually stored. Software which is required to do so anyway, cheats in this situation through a process called "dithering." Dithering creates acceptable reproductions of photorealistic images - if you don't look too closely - but they typically appear coarser and less detailed than they should.
At colour depths of more than 256 colours Windows is unfettered by palettes, and all the windows on your screen can use whatever colours they like. Photorealistic images can look genuinely photorealistic.
For practical purposes, the latter three colour depths listed earlier all behave the same way under Windows. More colour depth will actually display the colours in photorealistic images more accurately, but unless you sprung for a really good monitor and you have better eyes than most people sport after a few years of bombardment by cathode rays, you'll be hard pressed to spot the difference.
The amount of colour depth and the pixel dimensions of your screen are a function of the amount of memory available on your display adapter. If there's enough memory available to contain all the pixels required for the display mode you have in mind you should be able to configure Windows to use the mode in question.
The amount of memory required to contain one pixel can be figured as:
Multiply the two screen dimensions of the mode you're interested in together and then multiply this number by the number of bytes required for one pixel to arrive at the amount of memory your card would need to support it. Display card memory is expressed in megabytes - wherein one megabyte is 1,048,576 bytes.
For example, to display 800 by 600 pixels at 32,768 colours, you would require 800 x 600 x 2 bytes of display card memory, for a total of 960,000 bytes. This is less than one megabyte - this mode would be available on a display adapter with one or more megabytes of memory.
You can usually ascertain the amount of memory on your display card by looking at the BIOS power-up message which appears when your computer first wakes up in the morning. You might have to turn on the monitor a minute or two before powering-up the computer.