Upgrades - the first thing you should know is about the buffer assembly. That's the spring and the cylindrical object inside of it that resides in your weapon's butt stock. There are two kinds of them:
1. Full length buffer assembly, used in most longer or fixed stocks;
2. Carbine length buffer assembly, used in most telescoping or shorter stocks.
If you try to put on a stock that only has enough room inside it for the shorter buffer assembly, and you've still got your longer buffer assembly, it won't work. Likewise, if you're starting out with a short buffer assembly, it probably won't work with the full length type stocks. You can order either type for around $20 or so last time I checked. Got mine for $19.
I can recommend the Eotech holosight or Aimpoint 1x red dot sight for close quarters, but for anything else, I'd recommend a Trijicon ACOG, any model, with the 4x magnification. You can expect 3 MOA or perhaps better, so that will be enough to make the most of the weapon's accuracy capabilities out to 500 meters or so.
For a butt stock, I'd recommend any Magpul brand stock, mine is a M93.
I good muzzle brake always makes a big difference, and if you've got mad cash and a clean record, make it a point to get legalized and buy a suppressor. Sooo nice to shoot it with one on. I'd recommend Surefire muzzle break/adapters and their suppressor as well, but again, only if you're on a huge budget.
Then, get yourself a good 3-point sling and learn how to use it properly. If you're right handed, it feels great with the body strap going under your left armpit and over the top of your right shoulder, and the weapon will sit at a comfortable angle resting on your chest with the proper adjustment. Single point slings are pretty high speed too, but just not quite as stable.