I like to build massive topologies and the current linux VM I have running it with 6 vCPU is not enough so I am looking into building a hardware Linux server to host dynamips and qemu on. I use it as a remote server so I can run GNS3 from both my underpowered home computer and my very underpowered work computer and I typically have something running on both of them at the same time. Typically use the 7200 with IOS 15 so it tends to be a bit resource hungry.

Is the number of cores more important or the speed of those cores? Or is it all total number of clock cycles (4x 2ghz will be as fast as 8x 1ghz)?

when you say massive what it means? how many devices you want to run? for me a small topology of 20 routers just consume 1500 Mhz with basic config and eigrp running. It all depends upon RAM and a good IDLEPC value rather than very power full CPU.