The dual booting wont decrease your system performance. As only one OS will be running at a time.
However if you run one OS through VMWARE then the Host OS will share RAM and other computer resources with the GUEST OS. Resulting in decrease in performance. 
EDIT: I would not recommend to run OS on VMWARE unless you have 512 RAM or above and have at least 2.0 GHz machine (that too in the case where you are having one as windows and other as Linux).