Can anyone tell me what the VRAM info means (showed with -show_mem_usage flag)? I mean what DYN, POF, C-BMP, S-RAM and VRAM are.
DYN = dynamic memory, basically anything that's been allocated by the vm_* memory functions. This does NOT indicate total memory usage but does account for most of it.
POF = model memory, how much memory is being used by models (does not include textures)
C-BMP, BMP = bmpman memory, how much is used by the textures/images that are loaded. C-BMP means that you are using -cache_bitmaps and it will try to keep textures in memory between level loads and as the new level is loading it will keep what it needs and unload what it doesn't. Can greatly increase level loading speed, but does so at the cost of memory between level loads. This does not increase overall memory usage though, only during the loading screen. BMP means that it is not cached and all textures are unloaded from memory at the start of level load and all needed textures are then loaded from the disk again.
S-RAM = sound memory, how much is used by the sounds loaded in memory at the time. This counts static sounds only (from sounds.tbl, etc.) and not streaming sounds (music, voices, briefing stuff, etc).
V-RAM = approximate memory usage of video textures. This is not mean that your card has this much memory and can report higher numbers that your card actually has memory. For OpenGL it also includes some of the geometry data, but this number does not reflect the total memory that your video card is using. It only reports how much data the game has purposefully loaded into API memory (meaning it could be in system memory, video card memory, or AGP memory).
How can I know how much free physical video RAM and total texture, (physical + AGP apertured), RAM I've got?
You can't really. I think that Windows can sort of do this but I don't believe that it's all that accurate.