On the other hand there is a bunch of actions can be made directly from the 3G module without having to go through the Arduino microcontroller. This means you will control all the action from the main program running in Arduino although the data may flow from the 3G module to the SD card or from the video camera to the 3G module without going through the UART avoiding a possible bottle neck and ensuring maximum speed performance given by your 3G carrier (~7.2Mbps download, ~5.5Mbps upload at max). Some of this independent actions include:Uploading and downloading files to HTTP and FTP servers directly from or to the SDRecording and uploading video and photos to the Internet using the video camera connected directly to the moduleRecording ambient audio and sending to a web server
Ok here's the thing. The motion detection isn't what I'm worried about.
Perhaps you could use an Eye-Fi (WiFi SD card) in the camera and interface to it that way? Again you're going to want something beefier than an Arduino to receive the stream. Perhaps a Raspberry Pi?