The cost of the board is minimal compared to the cost of the hosting if you have a busy board.
We are only a mid sized board, and we transfer close to 300Gb/month, and thats no videos/audio or any other large file types. I have found that most hosting companies either charge a ton for that much transfer or its unlimited but you are on overloaded servers. We have a dedicated server 2.4 Ghz 1 GB Ram with 2MB/s connection to the internet with unlimited transfer. Its expensive but thats what is needed to keep this site running.
As far as hardware, I would say you need at least a 1.5 Ghz with 512 MB Ram (1GB preferred). Again that depends on how big you are talking.