For Giants hit "t" which should bring up a text box then type "fr" and hit enter, that should give you a real time FPS counter. I forget SS's
Screenshots, you can always use the "PrintScreen" key on your keyboard then exit out and paste the image into your application of choice. I'm not sure if either of the games have in game screen captures or not as Quake3 does(I'm almost certain Giants doesn't, SS I think does<shrug>).