Yeah, bear in mind a lot of the "buzz" for this movie (such as it is) is coming from pretty much the same crowd who three years ago thought Snakes on a Plane was the funniest thing ever, and would be the film event of the year.
I don't think its the same. Geeks on the internet went wild over Snakes because it sounded like a terrible/hilarious film, and they were celebrating its awfulness, which the studio mistook for genuine interest in the film; when it came down to box office most of these people couldn't be bothered to show up, or couldn't get mom to drive them to see it, and it turned out that most of the "hype" for it was ironic. I don't think many people are being ironic about wanting to see The Expendables; pretty much anyone who has ever seen more than three action movies is taking one look at the cast list and getting excited.
Yeah OK so Dolph Lundgren and Eric Roberts are only going to get a small group of people excited, but you've got Stallone, Statham, Li, and Rourke, not to mention an Ahnuld cameo (which they need to take full advantage of for the trailer), and those are all genuine names/stars. Plus you've got a wrestler and a UFC fighter in there, which should get at least a portion of those fanbases to turn up.
As for box office, I really have no idea, but I think foreign/non-US is where most of the money will be (both Rocky Balboa and Rambo did better outside the US). If it does Rocky Balboa numbers, it will probably be considered a success. Its funny though, Jason Statham is considered one of this generation's big action stars, yet his biggest solo hit, Transporter 2, grossed a whopping $43 million in the US. So if that's considered a hit and is sequel-worthy, then The Expendables should do just fine!