performance gain with new hardware

hi there
am debating investing in a new iMac or Mac Pro this summer, and am wondering how much of a performance gain I am likely to see when looking at Quad/Hec/Octa core options. Just how much can parallels desktop for mac make use of the additional hardware?

main usage is running server development environments - primarily sharepoint - on win2k3 & win2k8 guest OS's, with a virtual windows client in...erm.. parallel...