The memory is essentially impossible to upgrade. The memory is part of the SOC so you cannot change the memory without changing the CPU and GPU (The whole SOC).

In terms of USB you can technically hook up 127 devices per USB port. Using the USB ports you could also add more storage to your Raspberry Pi.

Also the Raspberry Pi foundation are also working on a camera that can be plugged into the RPi. Into one of the header ports, not using the USB ports.

Also I suppose you could use the GPIO pins to add just about anything to the RPi.

The memory is essentially impossible to upgrade, well unless of course you'd like to de solder the RAM from the SOC. The memory is part of the SOC so you cannot change the memory without changing the CPU and GPU (The whole SOC).

In terms of USB you can technically hook up 127 devices per USB port. Using the USB ports you could also add more storage to your Raspberry Pi.

Also the Raspberry Pi foundation are also working on a camera that can be plugged into the RPi. Into one of the header ports, not using the USB ports.

Also I suppose you could use the GPIO pins to add just about anything to the RPi.