Best trigger upgrade for the money, hands down. - JP Enterprises Enhanced Ignition Reliability Trigger Spring Kit

This spring set will take your MIL spec trigger pull from six to ten pounds down to four-five. If you put it on a polished and nickel-plated FCG you end up with a greatly improved trigger and reset for around fifty bucks.
These lighter springs do increase lock time (the time between the trigger pull and the hammer contacting the firing pin) with a MIL spec hammer which does negatively affect accuracy. But overall accuracy has been improved on every gun with MIL spec springs I've replaced with these. To get more accuracy potential from the FCG it takes a redesigned, non-MIL spec trigger like the Geissele SSA-E, JP Enterprises, etc., and that will cost 200 bucks and up. If you are happy with shooting two inch groups at a hundred yds. with good (American Eagle) bulk ammo then there's no need to spend that much money on a FCG. That is the accuracy limit of bulk ammo in all of my guns.
If you're after the highest accuracy potential in you're gun with high quality ammo and good magnified optics then it will take the extra money on a new FCG design. This is the most economic FCG upgrade that I have experience with. It goes on every rifle I build now unless I go with a high-dollar FCG for a match or long distance hunting rifle build.