My two cents
it's true that you won't get much for 400-500 but every now and then you'll find a gem at a good price. The price thing is mainly 'cause here in Canada we get royally screwed when it comes down to airsoft prices. Guns in the U.S. are way cheaper but also not Canadian legal (need to have a clear receiver) many players paint their receivers (I run an Aug that I got a great price on, thanks again Don) If you have some time on your hands look around and unless you're dead set on getting one within the next month. I'm finding a lot of players looking to thin out their collection and it's usually a good time to get a good deal on gear. There are a few companies that are really hit and miss with their products, if the gun seems to cheap to be true it probably is. You don't want anything with a plastic mechbox or plastic gears. I have several friends that run G&G guns, mainly M16, M4 variants They do quite well. Remember that you first airsoft gun (unless you get a kick ass deal on an upgraded gun) will be stock. Attend events and ask as many questions as you want most people friendly players will be more than happy to help you. It's not a bad idea to sit a few games out and watch how the gear performs from a distance, that'll give you an idea of range, speed, accuracy. Bring a note book 'cause you'll need one. Find out who shoots what and how long they've been shooting it for and what their thoughts are on it, ask them if there was anything they wish it did better.
I know it's a lot to take in but you've had experience with real firearms so you know that the real and airsoft world are far apart other than the look, ballistic details don't really apply here. Find something you like, play with it for 6 months and then evaluate (unless you get nothing but problems). I've been lucky I've spent less than $500 on a TM AUG with 6 mags, I've learned what it will do and what it won't.