Let's try a rough calculation of the bandwidth required
If we estimate that each user loads a page of 100kB every 30 seconds
500000 / 30 * 100KB = 1.6GB per second = you need 20GBit in connectivity :rolleyes:
So perhaps 30 high end servers (going to need plenty memory, good cpu and a fast disk array to serve a constant 1Gbps, and some spares for redundancy) at £500 each a month, then perhaps another 4 for load balancing, and another bunch for backup, and a database cluster..
I'd read 37signals 'Getting Real' online ebook, then reconsider prematurely grand hosting plans.