Attending ACEP isn't cheap, but nothing about interview season is cheap so I wouldn't let that be your deciding factor -- especially if you are concerned you may not get as many interviews as you need. The residency fair is worthwhile if you do your homework and have a plan of attack for the 2 hours of speed-schmoozing. You can see on the ACEP site which programs will be there and they are divided out by region. Find the programs you're interested in, shake some hands, mention specifics about why you'd like to rotate/interview there. Ask a question, show interest. Some programs have sign-in sheets and this can lead to a few points in your favor when they are deciding who to invite for sub-Is or interviews. You will definitely get business cards and contact info for everyone from residents to PDs. If you have time, go say hi at places where you already have sub-Is or interviews scheduled and re-affirm that you are looking forward to spending time at their program.
As in life, a considerable portion of this whole game is who you know and how to stand out from the massive pile of faceless applications. If someone on an interview committee remembers your name, it could help. If you have a resident you can follow up with ("Hey, remember me, we met briefly at ACEP") it could help. I absolutely got multiple interviews by attending. (Besides, you can secretly judge programs by the swag they are handing out. J/K, mostly.)
Residency fair aside, ACEP is a blast. Cruise the expo floor and play with all the shiny new toys, and go to lectures that interest you (they are all short and geared for EM attention spans).