Well, phpbb used to use a MUCH higher server load than the others but this has improved slightly (not enough for me, I run a heavily modified set of code to reduce the server load) and will improve in the future.
For most sites that won't be an issue but for a forum like this (1000+ posts/day) it makes the world of a difference.
phpbb's main strength is that it is used by many people who in turn develop many mods for it (e.g. I will soon release about 20 of my own modifications). The wide userbase is also good for support as many have run into all the problems you have.
vb is easily the best one out there but it's not open source.
phpbb will have attachments in the vanilla version shortly. Acyd Burn (the coder who wrote the attachment mod) is part of the development team and attachment support will be standard in 2.2.
But there are some others. I will be placing them here.
I'd go with phpbb if you want to really customize it and are not a PHP wizard. If you just want professional support and security go with vb (but for goodness sake make a fresh template as vb templates tend to be butt ugly).