When it comes to virtualization--and especially hosted virtualization like Fusion, where the VMs run on top of another OS--every little bit of speed/capacity helps. Especially CPU and memory. Also, I'm pretty sure any extra cache memory will help. Since the VMs can't access the hardware directly with this type of virtualization, there is overhead to consider. Remember also that these resources are pooled and abstracted for the VMs, so while your single VM running Win7 may not need 1.8GHz (which of course it will not have full access to anyway), having extra GHz allows you to run more VMs simultaenously. In addition, the pooled resources are dynamically allocated, so when your Win 7 VM isn't crunching numbers but your CentOS is compiling a kernel, you'll have a more pleasant experience). The concept goes for RAM as well. So of course it all depends on what kind of VMs and how many of them you need to run, but just remember that Lion needs its share of resources too.
I think storage would be a non-issue for most cases since thin provisioning the virtual disks is pretty much standard. You can over-subscribe your total physical storage space and not worry too much (until/unless your data starts growing exponentially).
Believe me: with virtualization, more is better.
