2
i can't give u much as regard to how it will cope.. in theory it should do quite easily..
but what % of the members are going to be actually a) online and logged on at once? b) how many people are u expecting to be viewing the site at the same time?
either way it's going to be a large amount, so i'd say a dedicated server is going to be the only viable option, 1 with a good cpu, lots of memory and lots of bandwidth..
i wouldn't even recommend trying to run a site as large as that on a hosted server..
have you got any stats to go on from yahoo etc? they should give you a rough idea of the bandwidth you may require..