Just to add on to what Josh said, as he is correct.
But if you are pushing only 3-5 gigs of bandwidth, I don't think you would be quite ready for a dedicated server yet. Unless somehow you were using a TON of CPU power yet not sending much data, that is definately within a shared server's limits for the most part.
My UBBT site was on a shared server for about a year before I was being hinted by my host I had to either upgrade or move. And at this time I was pushing 20 GB/month and had about 800 members with 30-40 online most of the time.
So off to dedicated.. or I guess semi-dedicated because I also hosted a few of my other low traffic sites on the server, and things did improve a bit as far as loading times are concerned.
But in your case, unless you have 50+ users online and are pushing 15+ GB/month traffic, I doubt you would benefit much from a dedicated server
