[size=150]I think a lot of casual players, especially when playing online, tend to get familiar with a very small opening book. I know in my games, (I"m a lower rated player btw) I tend as white to play d4, and as black, tend to like French and Indian defenses.
The problem with online chess is that player want to play alot of games and get rating points fast, so their opening book is very small. Too many players on FICS, ICC, Yahoo, and other online chess sites… tend to play fast or very fast time controls, so they want to get the opponent into thier opening book as fast as possible.
I’m now only starting to branch out in other openings like the Benoni and Caro-Kann for fun. Ya, I want to study the Sicilian sometime too. 
Back in the day when I was playing at a chess club (when I was in the Army like 20 years ago), not only did our small group of players had a very diverse (if not exactly cutting edge knowledge) of openings, but at least once a month we’d do a thematic meeting, and play some particular line for fun. (For example, maybe a specific line in the french or Roy Lopez, or maybe a less than popular opening like Kings Gambit or Tromposky). In any event, we didn’t get great at playing those lines per se, but it was fun as heck to walk in the door and be told we had to play some line nobody was prepared for. 
As far as needing computer preperation… There is alot out there, but nothing can help you more than good chess knowledge from level appropriate book. (not sure what books a 2000 to 2100 could use, but certainly I got a lot of help from Reasses Your Chess by Jeremy Silman. ~Need to go through that book again… I went through most of it, but due to RL stuff, had to stop playing chess for almost a year and forgot everything in the book. 
I’ll get back to it soon though.
There are certainly many ways to get your oppenent out of book pretty quick: Birds Opening, English Opening, Kings Gambit… those seem to be played alot less online (course I’m not level 2100 though).
As black, I noticed not to many people are familiar with the French Defence, Benoni, or Caro-Kann. ~Had some success with the Benoni, but I need to get some real material to learn the Caro-Kann, looking at a few games from the CM-11 database didn’t seem to help my games too much. 
I’m pretty familiar with various Indian defenses, but getting a bit bored with em. hehe.
If you want a free database program… Chessbase Light from http://www.chessbase.com/ might fit the bill. If you go to http://www.pgnmentor.com/ and click to the database section… that have scores of small databases (under 32000 games per database)… ranging from dozens of openings to scores of players, and many many tournaments.
The free version of chessbase light… you can’t open database more than 32000 games (or your limited to the first 32k entries is more accurate), and you can’t have mulitple databases open, and you can’t save/edit databases. But using Chessbase Light with the files from pgn mentor website, its actually quite a usable combination for free anyway.
Bottom line… you really don’t need computer preperation to play online chess, but stay away from fast time controls. Its really no different online than say… watching “Searching for Bobby Fischer” in Washington Park where the players are only using 2min to play chess and hoping for wild tactical gambits in lines that your not familiar with… only you can do it from the comfort of your abode. So just play slower times controls, use your noodle and have fun.[/size]