As they say on another forum, welcome to the "dark side"!

In addition to those boats already mentioned (I'm a big fan of the H-18) I'll add a couple to the list of boats to try before "stepping up". These boats are little more in horsepower and weight carrying ability.

Nacra 5.8 - these have acted as a great stepping stone boat in our fleet in the past. More hull volume and a better all around performer than the H-18, can be as tame or as wild as you sail it. Boom-less rig.

Prindle-19 - same comments as above, has centerboards instead of daggers, Tornado-ish in looks.

Hobie-20 - a little faster and more responsive than the boats listed above, but still a good option. You may still find pockets of Hobie-Fleet racing for it.

These three boats offer more performance than the other boats being mentioned, but are available affordably in the used market, with the H-20 perhaps the easiest to find. The H-20 is still available new from Hobie (last I heard, not roto-molded yet). As they offer more performance up front, they may be a little daunting at first, but there's more room for learning and you won't get bored as quickly. Spin rigs can be added to any of those, with the Hobie there is most likely a kit from the factory while the others would need to be "engineered".

Myself, I would look for the 5.8 first, but they can be hard to find. Next I would look for the H-20.

As with any cat starting out for the first time, pick your conditions right. Try to stay in about 8-12 knots of wind speed. Less than that and you might be frustrated, more than that and you might be scared. Get familiar with the boat and then start expanding your range. You'll be addicted soon....