After buying a rear iron sight, I would shoot the gun some and figure out what problems you need solved. You don't want to hang a bunch of gadgets that you don't need (not to mention spending the money).
I would recommend you consider the following:
1. A rear sight, folding or fixed.
2. A sling. My choice would be the VCAS from Blue Force Gear - Savannah, GA
, but there are alot of good options out there.
3. Some kind of flashlight mounted for low-light shooting. There are all kinds of mounting options, you don't need a quad-rail to mount a light.
I think these suggestions are going to cover about 99% of the issues you are likely to encounter. Before I spent to much money on other cool mods I would probably buy magazines and ammunition.
After that you can start adding trick-ninja mods that you like, as long as they don't take away from your rifles effectiveness. Some of those "neat to have, but not necessary" type modifications might include the following:
1. Hogue or ERGO replacement pistol grip.
2. Railed hand guard.
3. Vertical foregrip. I like the Grip-Pod, but it's expensive.
4. Replacement stock, such as Magpul CTR.
5. Ambidextrous controls like the safety selector and the AMBI-Catch magazine release.
There are also hundreds (if not thousands) of upgrades for the internals, from buffers, to bolts, to triggers, etc etc etc.
Here is a link to another good thread about extras & such. http://www.firearmstalk.com/forums/f20/rookie-looking-some-advice-10735/
Enjoy and have fun.