Steam quite likely will do a STALKER bundle or deal over Christmas which won't cost much. Same with Company of Heroes, and possibly Medieval II. (all great games IMO)
Shadow of Chernobyl and Call of Pripyat are the good STALKERS, or so I hear. Clear Sky: Apparently not so good.
HL2 is well worth playing, but lacks Iron Sights.
If you like a good deal of realism, I hear ArmA II and Operation Arrowhead are good.