If you do not have an existing PXE server and cannot modify your existing DHCP server, you can install proxyDHCP so FOG answers all DHCP client requests for PXE boot information with the information necessary to boot to FOG. Using proxyDHCP, your existing DHCP server handles all the IP address leases, but FOG also answers requests from PXEClient with additional information to allow them to boot.
There are some patches to FOG created by the community that allow you to select which HD/partition to image. Search the forums because I don’t remember right now.