Well, it not only depends on the traffic the site has, e.g. you should consider:
- What kind of script do you use? Does it have an "intelligent" caching system? Or are the pages fully or partially renewed every 1 / 10 / 100 hit? That would mainly effect the cpu load
- Do you plan to use mostly pictures or 50/50 pictures + textlinks or text only? That's important as the pictures will use most of your bandwith
Also ... you should think about the fact that you won't have 300-500k hits the day after you started ... maybe start with a "small" server, monitor both bandwidth and cpu/memory use, so you can calculate the appr. requirements for the size you're aiming and then switch to a server that will match this requirements?
Hope i could help a bit
