back to the topic, the features you should look for is:

-little nubs on the inside of the chamber to cath ice (or wtih a Zong or 'Z' you should be able to put ice in without the nubs.
-nice big bowl, and if the bowl is bad, you could most likely negotiate for a different bowl (don't get a metal bowl... point final)
-don't even bother looking at anything under 8 inches... you're better off with a bubbler then.
-this isnt that big of a deal, but try to get one of those 'o' rings that you can remove and put back on your own... my friends' bong (or the Kong, as we call it) has one and we scape the resin of it from time to time and have an nice free rez sesh.
-Range you should spend on a bong should probably be between 70-140$, any thing lower isnt really that great, unless you get a great deal on it (use your common sense)
-if by some chance you do get a plastci bong, you could probable get a 2 foot one for about $40, i don't recommend it, unless you and your friends are known for getting your paraphernilia confiscated (like another of my friends, haha)

Good luck finding your bong, bong shopping is always fun.