Newbie with a Trek 4900 (2001)

knownothing

After my trusty Trek 820 (that I got when I was 14) was stolen two years ago, I went to my local bike shop to replace it and found that they had a floor model of a 4900 in my size (I'm a shorty) that they were trying to get rid of that cost me the same as the 820 would have. Thinking this was a good deal and having grand plans of trying some off-roading, I went for the 4900. About a week later (in a last minute reshuffling of my life) I moved to Italy and left the new bike in parents' garage for two years. Now I'm living in Chicago and have at last retrieved the bicycle and I'm realizing that I don't know the first thing about it (I just discovered what a presta valve was when I went to put air in the tires). I'm wondering if anyone here is familiar with the Trek 4900--mine's a 2001 model if that makes much of a difference--and if ya'll can tell me what makes it different from my 820 from the early-mid 1990s.

Also, since this bike has been sitting (well, hanging upside down) in a garage for two years, is there any basic maintenance I should do or anything I should check before I head out for a ride?
(Anyone have a recommendation for a good bike shop in Chicago with a knowledgeable but not intimidating staff?)

I'm pretty clueless but I'd like to get the most out of my bike.