You should definitely be looking at a dedicated server or two. You can get packages where you have your own dedicated server, and pay a tech there to manage it, or you can find someone else to manage it for you. At that many accounts, and presumably with the amount of $$ it brings in, I'd be nowhere near a VPS.
And, yes, HostMySite should cut you a deal for sure, especially if you hint you might be going elsewhere if they don't. I know some good hosts, PM me if you're interested. I'm personally hosted @FlyingCroc, but they are kinda $$$ and I don't foot the bill
At $1500 a month, you should be able to get a dedicated box or two and find someone to manage it part time and there's a still a possibility of coming out ahead.