Bring down the console ingame by typing the ~ key.
There is no 'average' framerate counter, just the one that shows the current framerate.
type /cg_drawfps 1 and the FPS will show on the screen in the top right I belive.
/com_maxfps is the maximum FPS your system will draw, I would reccomend something like 65 FPS for a system like yours, if you have cable/dsl or better. This command can have a big effect on lag.