I am planning to develop a large forum and don't have any books on servers. How much traffic can a shared server handle at one time? When do I need a dedicated server and how much traffic can a dedicated server handle?
If you are starting your forum now maybe you wont need a dedicated server yet. A forum can easily be hosted on a shared account. I think that at the moment most dedicated servers can handle around 1000 to 1500 gb of monthly transfer.
ya definitely start with a shared account first until you really need your own server. You will know once you host starts to complain about resource hogging.
Forums can be really troublesome when they get popular on shared hosting and your host will let you know. Shared hosting can take a huge amount of traffic depending on what the site is doing, I have a shared hosting account that does 9gb per day and works perectly well but that would not work if the site was a forum.
You don't need to spend all your money at the beginning, but you should be monitoring things as your site becomes more popular. Better to see the need coming a month down the road than have to try and get everything moved in a day. If you foresee the need in the future to migrate to a dedicated server, keep track very closely of all your changes, and all the libraries/dependencies that are going to need to be set up on your dedicated box. Also, remember that not all dedicated servers are created equal. A very nice box with very nice bandwidth can do much more than el-cheapo, but el-cheapo can handle quite a bit of work too. When you migrate to a dedicated server, your community will see that you are doing positive things and it will generate good will within your community. You will know when it's time to move. You'll notice performance problems getting more frequent and your host will notify you. Personally, I prefer dedicated or co-located servers - so I don't have to deal with hosting company support issues, changing costs, etc. and I also get full control of things. It's an expensive way to start out though. I moved to my own server after having hosting problems and never looked back.
Another thing to consider in deciding if you need a dedicated server is configuration time. When you're popular you don't want any down time and my ISP needed two days to make the configuration changes. Two days when no one is visiting your site is nothing, but two days of down time for a busy server is an eternity which may loose you some visitors. Good luck. http://www.findingfacts.com
Most people completely over-estimate what they need for their site in terms of server resources. Whilst everyone would like to think they can build a great forum in a short amount of time, you are more likely than not to be using a shared hosting account fine for some while, unless you have a particularly significant advertising leverage to bring in heavy traffic pretty quickly.
I always start off forums in a shared account. I'd go broke if I started each one off on its own server. Having said that, please realize that by the time they become "huge" they are already a huge pain to move.