First I don't know where you are but in some states you aren't even allowed to have a modified car so even a 4AG sprinter is illegal.
Exhaust and extractors will give you a bit more power but nothing too exciting. I think things like cams and high compression pistons are where the big powers at. Also depending on the health of your 4AG a rebuild to freshen it up might do a lot for power.
I'm sure some of the gurus here can tell you how much you'll get from what and give you lots of other things to do. It just depends how much you've got to spend and how much you want out of your engine.