No problems !
Be aware though that the Pi doesn't have the greatest bandwidth transfer capabilities.
2 NICs will be sharing the same USB bus so if you have super fast internet, the Pi may cause it to go slower.
One of the appealing things on the Pi is that you can do anything with it, which of course makes things a bit more complicated.
If you want an easy guide on what you can do, I recommend the MagPi e-zine. Have a look through the issues and you can get a good idea of what to start on.
Yes, it's possible, but like plugwash said you need to specify which traffic you want to count. You could conceivably monitor *all* traffic flying around your network if you had a switch with port mirroring Or you could monitor all traffic between your network and the internet if you put the Pi betw...