VGA double buffer with block RAM
The external SRAM on the MicroNova Mercury board is a fast one &mdash; 10 ns nominal speed &mdash; so there should be no problem accessing it at 50 MB/s (or more). You could easily read out a full VGA signal at a raw rate of 25.175 MB/s and still have half of the memory bandwidth available for writes. Of course, the SRAM does not have enough room for two 640&times;480&times;8 bits buffers, so you'll have to cut back somewhere. If you do 320&times;240 @ 60 Hz, you'll only need to read out 4.608 MB/s on average, leaving more than 90% of the memory bandwidth for pixel writing and other purposes.

I see no need to use internal BRAM for pixel buffering; it's a resource that you will find much more useful for other things, such as implementing the controller(s) that will be running the game logic and drawing the pixels.