Hi there, your best option for a website receiving as much traffic as yours, would be to stick this in the Amazon Web Services cloud.
What i would propose is that you create a mini WordPress autoscaling cluster, that would reduce in number of servers over night and increase at peak times, to ensure your end users receive a fast experience and that you don't pay an arm and a leg for a powerful dedicated server when your only utilizing its power 25% of the time.
AWS has some fantastic tools including hosted mysql. They host the database for you, manage replication to other data centres for backups. Manage snapshotting of your data, meaning to you can concentrate on your website, and not worry about maintenance and backup's.
Im not a fan of sticking all of my eggs in one basket, a single dedicates server is not the way to go! A autoscaling environment in multiple Amazon availability zones would also make your website highly available. You don't want your website to be down if a data centre has a power outage (yes this happens!). We can also implement a load balancing frontend to ensure that user load is spread across your cluster.
I would also offload your static resources (images, CSS, JavaScript and other media files) to Amazon Cloud front. this is a content delivery network, which delivers the large files on their network as opposed to using up your server resources shifting large files. (have your web servers serve the content and do the processing.
Last but not least, it is important you take this step towards a scalable solution now, especially If your website growth rate continues to grow. you don't want to have to move to another host in 12 months time and repeat this all over again. moving host is a pain in the ass! with amazon web services, we can create you a highly available and scalable infrastructure, meeting your needs. in 12 months, we simply increase your resources, at night, we start turning things off to keep more money in your pocket, not your web hosts.
I would love to discuss this with you in more detail.
Kind regards
Steve