Any OS will reserve certain chunks of RAM for its own exclusive use. Some of that - usually about 0.3 to 0.4 GB - is used directly for various OS functions. Some of that is actually used for storing and retrieving data the OS uses. Some is actually memory mapped. That is, the actual RAM is not used, but data destined for that RAM address is actually sent to separate RAM on some peripheral card, usually its I/O buffers.
The biggest single chunk of RAM reserved is for the video system. If you have a simple system with on-board graphics that uses system RAM for the video, then that actual RAM is used directly for that function. But even if you have a video card with its own RAM, the OS still has to be able to get at it by specifying which video RAM locations to read and write, and this is limited by the OS's ability to generate addresses. With a 32-bit system, the addressing bus is only capable of working with RAM located up to 4 GB. So the physical RAM on the video card is used, but to communicate with that RAM, the OS maps a portion of the range just below 4 GB (in terms of its ability to generate addresses) to that physical RAM. This means that same address space cannot be used to access the main RAM on the motherboard in that area, and it is not available for any use. If you have 4 GB of real mainboard RAM installed, that video RAM size appears to be used as RAM reserved by the OS, also. So if you have a video card with 512 MB of on-board RAM, that 512 MB appears NOT free for use by anything else. Add this to the .03-0.4 GB already actually used by the OS, and your "lose" 0.8 GB or more from your real 4 GB of RAM.
Now, suppose you have 2BG of RAM installed and that big video card with 512 MB of its own RAM. The 32-bit OS will still reserve 0.3 to 0.4 GB of RAM for itself, but it may be able to map the 0.5 GB of addressing space still required to reach the video RAM to up above the 2 GB position. So, although it is using 0.5 GB of addressing space for that, it is NOT using addresses that overlap the real 2GB of RAM. You appear to have about 1.6 GB or more "free", so you've only "lost" that basic 0.4 GB. But when you put in 4 GB instead of 2, there is no way to avoid having the video RAM address space overlap with main board RAM address space, as long as you are still using a 32-bit OS.
Switch to 64-bit OS and the picture changes drastically. Now the addressing capabilities are huge compared to the actual RAM one can find and install. Even with the artificial hardware limit of 16 GB referenced in the link provided by Maverick2002 above, right now you can't buy and install more RAM than that anyway. So now you have two options. One is to stick with 4 GB of main board RAM and set it up (just like a 32-bit system with only 2 GB RAM) so that the video addressing is mapped is to address space above 4 GB and it does not overlap with real RAM. The other is to get big with RAM (like 8 GB) and let it use that upper 0.8 GB or more and just don't care because you got lots, anyway!
For your original question, you can try this approximate way. Windows will reserve for its own direct use about 0.3 GB, sometimes a little more. That amount will be used no matter how much RAM you have installed. IF you have on-board video using main mobo RAM, look for the specs' of how much main RAM the video chip uses. That also will be used, leaving you with whatever is left over as free RAM space. However, if you have an added video card with its own RAM, how much impact that has depends on how much total mainboard RAM you have installed. If you have no more than 3GB of main RAM, chances are your OS will map its video card addressing range into the area that has no main RAM anyway, so you won't appear to lose any more than that 0.3 GB. But if you have 4GB of mainboard RAM, the amount you "lose" because of mapping addresses below 4GB (in addition to the 0.3 GB or so already used by the OS) is exactly the amount of RAM on your video card.
For a practical example, assume you are using 32-bit Windows as your OS and have a 512 MB video card installed. If you put in 2 GB of main RAM, you'll actually have about 1.6 to 1.7 GB free for application use. Don't forget that any application you have set up to load automatically every time will cut into this space. If you take your machine up to 3 GB installed, you'll have about 2.6 to 2.7 free. If you go to 4 GB installed, you'll have 3.1 to 3.2 GB free. So, going from 2 GB to 4 GB installed actually will pretty well double the free RAM space. What about the in-between option of 3 GB? It's a whole 1 GB more than the starting point and only 0.5 GB less than the max. But at today's pricing, 2 x 2GB or 4 x 1GB of RAM could be the same price as 2 x 1GB plus 2 x 512 MB of RAM.
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