It is still geared but I am thinking of making it a single speed just so there is a few less things to worry about.
I highly recommend a clinic. They are informative and fun...at the same time. CX Magazine just posted some clinics in their calendar of events. Check it out and find a clinic in your neighborhood.
In addition to all the recommendations to do a clinic, I'd say just get out there and do it! It's a great time and of al the race scenes, I think 'cross is the easiest to get into. The racers and promoters are generally very helpful and encouraged by first timers and the spectators (usually other racers) are very supportive.
With your current bike you mention the tires fit but it's a bit close. You'll want to be careful of this because if you hit a rock or root and it knocks the rim a tick out of true you cold end up with some tire rub. Not to scare you, just to make you aware. The method you are pursuing is a common one (converting an old road bike) and as Oliver has mentioned, if you've been riding geared up until now, I'd stay with that for your first foray into 'cross.