Decode VGA port by Arduino
Calculation:

320 x 240 resolution (assuming 8 bits for 256 colors) is 76,800 bytes

Assuming a frame rate of 50, that is 50 * 76,800 = 3,840,000 bytes/s

The most Arduino's are 16 MHz so assuming quite some instructions are needed (for converting and doing something) I doubt you will get this frame rate.

Also, you cannot store a complete page, but this may not be necessary.