We can go to the moon with our current tech.
Block II SLS has about the same up mass as Saturn V (140Mt)
Block 1 SLS/Orion will be doing an unmanned lunar flyby circa 2018 currently.
The reason we haven't is entirely political. Back in the 60's going to the moon was a national priority and the budget NASA recieved reflected that. It was a budget that allowed us to be mostly complete within one administration, (JFKs death not withstanding).
Compare to today:
NASA recieves enough money for basically two manned programs. In the past it was fly shuttle and develop ISS. Now it's fly ISS and develop Orion. However the budget profile is such that it takes longer than one administration to complete. Budget priorities can be changed each year by congress. This makes it very difficult to complete a program, (e.g. Constellation).
As for the benefit of manned space flight, DrPizza and I have had this discussion at some length before. Unmanned space flight has a much better science bang for the buck than manned missions. Manned missions do return science but not nearly as much nor as cost efficient as unmanned.
However manned missions do provide a number of engineering innovations and spin-offs. I remember a comment from Chris Craft the original flight director for NASA. When they started Mercury they could get 22 Teletyped words per hour from around the world. By Apollo they had world wide global communications. Solving the difficult engineering problems of manned space flight and to a lesser extent unmanned space flight provides solutions for problems in other industries.
The other main thing it provides is inspiration. Basically every engineer I knew in college became an engineer because they wanted to work for NASA or wanted to be an astronaut as a kid. Most obviously didn't go on to work at NASA. Instead they went on to work elsewhere. How much benefit does the US and the world derive from kids going into science and engineering because they wanted to be astronauts?