RE: My Dlink DIR-625's Port Forwarding Just...Died :p Try to set yourself into the DMZ, if that still doesn't work you need to exchange your router. Re-flashing the firmware could work, but as D-link sucks it may brick it in the process.
RE: Wireless N questions 1. Yes 2. Yes 3. Yes, but not as far away as a regular 802.11n 4. Depends on your network usage, usually 802.11n is used for media streaming as you will get around 300mbps instead of the 54mbps of a 802.11g or 11mbps for 802.11b