If I were allowed to spec my work Macbook, then a 250Gb SSD would be top of the list, with 8Gb Ram.
But then running 4 - 6 VMs at various times is a bit of a performance drag and the SSD would help enormously.
Other than playing "Total War", cavortingcheetah doesn't tell us what sort of use the lappy would be put to so it's difficult to advise more.