Scalable hosting at a reasonable price?

Hey there,

I started a web design/dev company and we have about 30 websites for out clients which we host on a cheap reseller hosting account.

It's shared hosting and we like it because it's unlimited disk space, bandwidth, emails etc. We just pay for a set amount of cpanel accounts - 20, 30, 50 etc as we grow, and no hassles about managing clients disk usage or bandwidth, but it is limited on resources so not the fastest sites, especially with wordpress.

We'd like to move to a more powerful server that will give us really fast sites, and have it really scalable and easy to manage..

What would you suggest?
Is there a feasible way to manage this with Amazon E2?

Thanks!