Best Memory allocation and management

Dear Members:

Planning whether to go for for Windows 7 Ultimate 64-bit or Windows 8 Professional 64-bit (as host), keeping in mind better management and maximum possible memory that I could allocate (out of 8 GB) to VM Workstation, whose version not yet decided but definitely above 6.

My Dell Inspiron 1545 has 8 GB available memory. Any advise would be greatly appreciated .

Thanks in advance.

Atanu