I have an 20" A2 modified to service rifle specs. The former owner used it for competitions. I really like the bone stock feel of the gun, but seeing all the decked out AR's on here makes me wonder if I should add some extras to it, such as a rail system, a new stock, flash hider, etc.
Do these aftermarket parts increase the functionality of the rifle enough to really justify the purchase, or are they just fluff?
I'm in college, and I don't have enough cash for a new AR at the moment. I can afford a few hundred in extras, though. Is it worth it?

Few hundred bucks?

Mags and ammo.........I tend to follow the KISS train of thought....for ME, a $200 rail system is just a way to spend $180 too much on a set of handgaurds.....but that's just me....

I would go with a free-float system, not necessarily a rail system. If you like the rail and want it, definately get a free-float rail.

The free float is what I would consider as one of the  most important performance enhancing upgrades available. It allows the natural whip of the barrel to take place un-inhibited and in almost every case improves the accuracy of the firearm over a range of ammunition.

Just don't mount your sling to the barrel mount on the front sight housing mount. It kind of defeats the purpose of the free float. I cut my sling mount off and filed off the tabs flush with the bottom of the front sight housing.

After free float, I would consider optical sights as the next upgrade.

You can improve function a lot with a good trigger and a FF tube. Unless you really need a VFG, flashlight, laser etc., the rail forends are just expensive fluff IMHO . A good optic is also a great accuracy multiplier. An adjustabel M4 stock is good enough, and an improvement over the A2.

The 15 minute trigger job is a free upgrade.

To me, to modify or not depends on what you are going to use your AR-15 for.  For me, I know that I have my AR-15 for HD and SHTF scenarios so I looked at the accessories that would aid in these situations like weapon lights and red dot sights.