Hi Mark, it can be a little subjective, but as far as running Act! through a VM, one of my colleagues is using an early 2013 model 13" macbook pro, with a 256gb SSD, 8gig of RAM and running VMWare as the Virtual machine software. He has no difficulty running Act! in this configuration, and will usually have one or 2 other VMs running at the same time without issue.
My Biggest recommendations would be:
1. Go for An SSD drive
Act! is HDD intensive, so it will perform much better on an SSD
2. VM Ware for the VM Software
It is similarly priced to Parrallels, but gives you a little more freedom.
8gig of Ram is ok, but 16 Gig would be better looking forward.
Kind Regards
Jared