"Using DHCP" / "Using DHCP with manual address"

I have been looking for an explanation of the difference between these two settings. We have a network at home with a bridge DSL modem and a Netgear WGR614 router. On the network we have a Brother printer, a PC desktop and an iMac G5. Currently, I use the "Using DHCP" setting to let the router assign IP addresses to the various devices. We have added iSight to our iMac and to have video chats we had to open various ports on the router at a specific IP address for the iMac to establish 2-way video chat connections. However, when the iMac goes into sleep mode and then wakes up, the router always assigns a new IP address to the iMac which means the iMac can't do a 2-way video chat. In the router configuration I can reserve an address for the iMac which I attempted to do, but the iMac still is assigned a new IP when it wakes up. I am wondering if I use the other setting in preferences "Using DHCP with manual address" would the problem be solved? I currently don't allow the iMac to sleep so that the IP won't be changed. Would changing the setting mess up the rest of the devices on the network?
Thanks, Richard