Quizes consume lot of ressources. Still 2500 SQL querries per user sound a little high. Can anybody confirm the order of these numbers?
About your next option "virtual" server: Is it a virtual server as Apache understands it (one physical server, one Apache instance responding to different domain names) or a virtual machine (a physical server devided into comparments).
In the first case each server can go upto the peak performance of the physical machine, provided of course, the other virtual servers are idle. In the second case each virtual machine is capped.
Dedicated servers (one physical server for you alone) is not that expensive these days.