Unused RAM is a waste of, er, well, RAM. Linux will allocate RAM for cache and buffers when it has more than it needs.

Conky actually has two variables called "memfree" and "memeasyfree". The latter includes memory that is easily freed up, from buffers, etc. Note that the default Conky config uses neither of these.

A quick experiment with both leads me to conclude that the RAM Usage shown in Conky is based on the memeasyfree context, i.e. it doesn't include RAM allocated to buffers, etc.

In general, there's not much to worry about with RAM usage. If your Swap usage is high, then the system is getting heavily loaded.

Hope this helps.