Going from 4GB to 8GB on my mbp made a huge difference running VMs. Also, get Fusion. For working in VMs all day Fusion has worked better for me over VirtualBox.
You should try out Parallels then, as I find it much snappier than Fusion. After having tried Virtual Box, Fusion and Parallels, I kept the last one as it provides the best experience of the three imo.
Going from 4GB to 8GB on my mbp made a huge difference running VMs. Also, get Fusion. For working in VMs all day Fusion has worked better for me over VirtualBox.