How to stream video (and process) into FPGA

Hi,

I want to take a camera... (IDS camera) and stream video into my FPGA.

Camera interface will be GigE or USB3 depending on FPS i will decide.

in the FPGA i would like to do (what i believe is) simple image processing of... lets say frame subtraction i.e. subtract frame k from frame k+1.

resolution: ideally i would like to work with 640x512 with 300 FPS gray scale 8 bit.

I know its a lot of data and eventually i will probably downgrade significantly the SPEC but this is the ideal parameters i would like to work with.

I have a few questions:

1. Interface: how do stream the frames into the FPGA?

what component should i look for in order to take the GigE or USB3 output interface of my camera and connect it to my board so i can do something with it?

2.I assume i will need an external DDR? can you give me some pointers on how to choose what i need and the main principle on how to use it?

Maybe 2 DDR's and erasing the older frame every a new frame comes along?

3. can you give me an advice on which FPGA should i choose? i worked mainly with LATTICE so far and want to cross to ALTERA.

4. any suggestions/ reading material/ comment will be most welcome

Thanks!