LAST EDITED ON Dec-28-12 AT 12:49PM (MST)[p]First and foremost you have to decide what configuration you want. Everyone has different tastes. I recommend learning what the differences are, then making an educated decision.
Chambering
The 5.56 NATO will allow both 223 and .556 but its rarely cut as a match type chamber. The .223 Wilde chamber will allow you to use just about anything bullet weight wise, but is typically closer to a match type chamber that gives better accuracy.
Barrel length
Each has pluses and minuses. Shorter barrels are more compact, lighter weight (allowing you to go with a heavier profile to keep the same weight), but you give up some velocity.
Barrel twist
If you want to use a large spectrum of bullet weights the faster twists will serve you well. A 1 in 9" twist will shoot bullets up to 70 and maybe 75 gr depending on the gun/velocity. A 1 in 8" twist will get you up to the 80 gr bullet weights. Just because a barrel has a fast twist does not mean it will not shoot light weight bullets accurately. I have a 1 in 9" twist .223 that shoots 50 gr TNTs into bughole groups.
Barrel profile
Its up to you. How heavy do you want the gun to be? Just like bolt guns heavier barrels will have a higher frequency as they vibrate. The less they move the closer to the center they are.
Bore
Some of the ARs have chrome lined bores. This reduces erosion from the hot gasses from the ammunition. Typically match barrels are not chrome lined. Most chrome lined barrels will shoot 1.5 MOA or so at 100 yards. Another option is nitrided barrels. These have been soaked in molten salt and are extremely hard. They can make some nice barrels.
Muzzle brake or flash hider
Do you want a flash hider or a brake? Some do both but not all. The flash hider is to reduce the loss of night vision by the shooter due to muzzle blast. The flash hider mixes in air to cool the exhaust gasses to reduce the flash. A brakes primary reason for being is to reduce recoil. True the 223 Rem does not have much kick but a good brake will keep the muzzle flip down to see the hits on target.
Rails
Rails, foregrips, bipods, etc are definitely not necessary. It kind of depends on how tacticool you want to look. ;-) None of my ARs have rails on the forend. One does wear a bipod though.
I recommend getting at least one AR lower ASAP. That is the only controlled part of the gun with a serial number. Once you have the lower you can build or buy many different types of uppers to go on the one lower. You can go from .22 rimfire up to the macdaddy .50 BMG on the same AR lower. You can mail order anything except the lower receiver.