Your own dedicated server should not have a problem handling that amount of usage. You could always contact some hosting providers and let them know of your situation, and then ask if they would be able to house you, and an estimate on the price.
Have you look at optimizing your server? I'm not too big into it but if you haven't already adding stuff like Cache (ie. Memcache/APC) and Sphinx Search Engine would certainly help with your load. There's also other ways to tweak your server so that it runs faster. If you also want less on bandwidth, Google PageSpeed tells me that your images could be losslessly compressed (in some cases, up to a 90% reduction). You should consider running your images through smush.it.
We are in a catch-22 with upgrading. None of our themes will work with the upgrade, but we can't design a new theme for the final version until after the upgrade. I'm going to be designing it quick as I can over the next few weeks, installing it and taking my chances with the upgrade then.
As for hosts, we already installed Sphynx and such...we are moving to a new host called Softlayer tomorrow. Things just didn't work out with our current host and I don't fault them--they just seem meant more for sites that don't place such huge demands on the servers.
At least now we will have room to grow.
Is Alex still on this forum? I am trying to get in touch with him.
Well, they are our host now, and we're giving them a shot. Porting everything over now. I just need to track Alex down if possible.
After dealing with what I dealt with today I prefer to try hosting it myself directly on a data centre. By the way they even installed Cpanel and a firewall for me, they're great.
A Xeon quad core server. Really I'll be with them a while? Well, I don't put stock into a host now until I've been with them at least a few months. I'm cautious now, this is our 3rd host since we moved and it's only been a year and a half. They say it's about the journey not the destination, but I dunno 😛