I’m using Win 8.1 x64 and have 8 GB of ram.
And I noticed that with normal use i have (according to resource manager): about 2400 mb in use, 2676 mb standby (cached) and 2976 free.
In Win 7 and up having “free” ram is kinda bad (it should be cached), so do I have a problem, or Win 8.1 doesn’t have enough to cache? I use SSD for system drive, if that matters.
It is absolutely perfectly fine, what programs are you using that makes you think all your RAM should be cached? On a pretty much clean install of 8.1 x64 you will have after boot ~900 to 950MB in use, probably over a gig Standby and the rest free.
Windows and pretty much most systems (desktop) have now got to the stage where RAM usage is of minimal imact and is not even worth the trouble to look at unless there is a memory leak in a certain application. The only time you may have to worry about memory usage is if you have very little or are using very memory intensive apps like CFDs or games and even then windows with an SSD is pretty much the same. Dont worry about RAM, if you see someone say an AV is heavy because it uses abc amount of RAM just think about what at its basic level an AV does! That`s right it reads from a list of signatures to cross check against the given file. So which would be faster? Reading the signatures from RAM or from the HDD :-X
Crumbs it wasn`t long ago we were running everything off 256MB of RAM if we were lucky 
Good luck…