1 more vote for the EM1. I got a good deal on mine, 10k w/ 39k miles in hawaii! lol. It wasn't beat on, and it was a friend of mine, so I knew it was taken care of. I really really like my car. I don't forsee selling it ever. I love all the "luxury" features, moonroof, stock tweeters, keyless entry, and all that. Obviously DOHC VTEC is great, I'm still not sick of "spirited driving" after a year. I think they are one of the best looking usdm civics, and have so much potential.
One of the best things about my car is that I get 30 - 32 miles a gallon, even when I drive like a maniac. I commute a lot, and it is so great that I get such good gas mileage while having fun at the same time.
It's true that you could convert your dx, but that is a ton of work with potential hidden cost. I would buy the si, and in your spare time, start upgrading a solid base. That si on autotrader looks like a good deal, but definitely do your homework on it (watch out for those salvage titles!). Welcome to CC, you'll learn a lot about your car if you put the time in.