my con experiences
It looks like you're talking about this con:
http://www.wizards.com/default.asp?x=rpga/conventions/wf2005faq
It looks great, if you want to try out RPGA gaming. I'll give you the low-down on my con experiences generally and with RPGA specifically as well as some advice.
I had been gaming about 15 years before going to my first gaming convention. It was a small con in a small city. I ran a game. It ended in about 15 minutes with a TPK. What a disaster.
It was about another 7 years before I went to another con. GenCon. In Milwaukee for the last time. What a great time. I went with a friend. Highly recommended. I wish I had done a few things differently. I wish I had played more games that I would not ordinarily play. I wish I had played in Gary Gygax's game as my friend did. I did meet him, so that was cool. I found RPGA gaming unsatisfying. It is a bit too much work for too little reward for me. Overall, a great experience but a little to expensive with plane fare, hotels, etc. to repeat as yet.
In the past year, I attended 2 more local cons. The first was about 2 hours away. Smallish but relatively close. I drove over with a friend. Again, highly recommended. The RPG we wanted didn't run as scheduled, so we played a couple of card game demos. Very fun to stalemate expereinced players as novices. Then, we watched some of the events and surveyed a very wild hotel party before leaving. A good time overall.
The second was about 1 hour away, and I went by myself. Smaller but even closer. I played a couple of fun RPGs that I would not ordinarily get to play. Once it looked played out, I left and got home early.
One thing I always enjoy is the dealer room. Even at the smallest, most recent con, I picked up a used RPG book that I read cover-to-cover. I hope to do some trading next time. There are always good deals to be had.
Yes, you will encounter new smells, sights and sounds. It's just part of the territory.
You will also probably find these RPGA games to be well organized and attended. RPGA games were full and on-schedule at my local cons this year.
My advice is to go with as many of your gaming buddies as you can. Play games you do not or would not ordinarily play. Spend as much time and money in the dealer room as you can (if there is one). Find the parties. When it looks played out, leave.
I would go for the d20 Modern game first since it a campaign kick-off and you therefore won't be playing alongside players who have many years in the game with much knowledge and multiple, powerful characters. If you like it, you can contiue to play and build your character. Also, The Game Mechanics are awesome; so it will probably be a great game. As an added bonus, you can review the d20M SRD for free online to get a grasp of the rules.
Second choice for me would be the Eberron game. Also a campaign kick-off with benfetis noted above. WOTC's new setting has to have some appeal.
Living Force is also appealing. It's Star Wars, after all.
Living Kingdoms of Kalamar is good. That world has some nice features that make it appealing for a D&D game. I played a half-hobgoblin, which was cool. Only got 100 xp, though; so I probably wouldn't repeat it before other options.
I'm jealous. This con is only 30 minutes from your home. You owe it to yourself to go. Have fun!