Many routers allow to always pair a specific IP address to a specific MAC address, so if you know the MAC address of your Pi box and remember which fixed address you assigned to it, you could try to instruct the router's DHCP accordingly.
If you don't have the MAC address, check to see if your router stores the last-known pairing IP/MAC address DHCP did. and get the MAC address from that information. Else you could try to inspect the logs of your router to see if you can find that info...
If any of the above doesn't work for you, as a last ditch effort, you could try to disconnect completely your PC from your network (make sure you remove the Ethernet connection to the router AND disable wifi). Once this is done, you could try to see if you can setup an ad-hoc network between your PC and the Pi, by connecting the two Ethernet ports with an Ethernet cable and see if you can force your PC to act as a DHCP server.
Or - and this is definitely going to work
- switch off the Pi, reflash the SD and start from scratch - remember to then perform a git pull from /var/www
EDIT: another option would be trying to use a Ethernet to USB adapter, hoping that it works out of the box on ArchLinux. This should provide a new MAC address: the DHCP server on your router should then assign a brand new IP address to this Ethernet interface and you should be able to connect to the Pi box by using this new IP. Quickly reading through some posts, is said to be working on ArchLinux out of the box.
