I'm always staggered that people sell that spec of machine as a server - it's got a desktop CPU, desktop disks and a desktop OS - I'm sure it won't have dual PSUs, any form of lights-out management, cheapo NICs etc. But I suppose it's cheap I guess, I hope you're not betting your business on this server being highly-available
Anyway if they really mean 'unlimited' then you should be able to get between 50% and 90% of your NIC speed - so assuming you get a single 1Gbps NIC then you're looking at between 50MBps and 90MBps (from memory, those disks won't consistently get anywhere near that), which is 3GB/min, 180MB/hour, 4.3TB/day, 130TB/month (or so) - although I think if you were doing that kind of bandwidth all day every day your provider would soon find an exception to their 'unlimited' policy. Also that 256Kps for international traffic is REALLY low, I can imagine a good chunk of that disappearing for various reasons before you get your first international hit.
Now onto your first question, how many visitors, this of course depends on average page load - I don't know those ad sites you mention but presumably they need a DB on them to keep track of sales, and financial type activities plus the relatively lightweight work of dealing static images and text. If you need a DB on this same box, and that's a bad idea for a lot of reasons, then you're going to need more memory, seriously. Have you got your application running in a proof-of-concept yet? if so you should be able to use it to measure load and extrapolate - it's the best estimate you'll have until you're in production. What are your 'get-out' routes if this proves to be too much for this single server, can you spin up a second box quickly enough?