3.7g out of 4g useful in windows 7 x64. What is the reason of such memory gap in my system? Video uses 128M of RAM.
3.7 out of 4 is not that bad, the problem could be worse depending the other hardware you have.
It basically comes down to your hardware - including capabilities of your BIOS, as well as other hardware devices in your system which make Memory mapped IO reservations in the physical address space you would want to use for RAM.
I'm not sure why hardware devices still do this, but some are much worse than others. My old Dell lost over 700 MB to "Harware reserved", but my new one has less than 14 MB "Harware Reserved." You are right in the middle. Honestly I wouldn't worry about that now that you know why. Any quailty Hardware you buy in a few years will handle this much better.
You can cut to the chase reading this blog entry: Memory Mapped IO reservations reduce the usable RAM If you still want more after reading that one, move on to this How windows handles physical memory this one tells you how to spot the offending devices using device manager view "Resources by Connection"