BE sure that isakmp nat-t is enabled on your 506 as well as Site1. Also, if you have an access-list on the inside interface of your 506, be sure they allow esp traffic to the public IP address of the Site1 506. Since your tunnel is being built, this seems to be an esp issue. Make sure that esp is allowed from your PC al the way to the remote PCs. Since RDP likes to use enormous packets, this could also be a MTU issue, Can you ping the remote PCs? If so, try lowering your MTU on your local PC to something like 1100. If you cannot ping the remote PC and your tunnel supports split-tunneling, check the "allowed networks" or "secured routes".
Hope this helps.
Bryan