Scalability / Simultaneous Users

Hi,

I was wondering how scalable wordpress was, and what type of server configuration would be recommended or needed if my site is going to receive massive traffic.

I have used wordpress to create a number of small-medium sized sites, they do between 1 and 15gb of transfer per month and have between 0 and 40 people viewing at any one time.

I was asked to supply a quote for a site that the company believes will have approximately 5000 concurrent viewers at almost all times, as they plan to launch marketing plans across the globe promoting the site, tv ads, radio ads, etc.

I wasn’t sure if wordpress could handle that and if it can, what type of hosting configuration I would need or is recommended.

The above mentioned sites (about 20) are all hosted on a dedicated server running RHEL 4.

Thanks for any help provided.