Before use the warranty, try to put it at 0%. Then try to charge it to 100%. It may work. (You can wait a complete night for it.) I had a problem with a new Iphone once, I used the warranty to repare it, they didn't want to repare and they said I modified my Iphone. So be carreful with Apple...
That's strange. The price are usually going down very quickly for the memories stuff. Maybe you bought it during an offer for christmas ? Usually, all the 6 month, their is new stuff more powerfull, so the price should decrease.
Actually, 16GB is very big. 8 is "normal" actually and 4gb is really low. Developers wants to touch the biggest community as possible. So actually, I don't think it's useful in 95% of the games.