This is generally my thought as well, but most of my family and friends tell me this is the only time I have any leverage and that asking for more is expected and demonstrates you're not afraid to ask for what you want. <.<
Based on the fact that they already gave you more than the initial job posting I doubt they expect it. In fact the various HR people I have talked to seldom expect or want a negotiation. They give you the offer, you accept it or they go to the next person in what is probably a pretty long list.
You can try and negotiate, but if they turn down your attempt stop immediately. Negotiating against an already great offer could just make you look greedy and could cost you the job. Doesn't seem worth it to me, not when they already gave you more than you expected.
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(Maniac64 at work) [Browncoat] ~Board 570901~
All the proteins, vitamins, and carbs of your grandma's best turkey dinner, plus 15% alcohol.