If you need 64K and ADC0809 then you need bank swiching. Or if yo can do it with 63.9K of ram and the ADC

You must calculate the time to send 64K to PC because if you wait to complete the 64K and then send the data may be yo overload the RAM with new datas fro the AD. May be you must start sendig before the 64K are full.

For storing data you can do an FIFO stack with two pointers of 16 bits, one for input and other for output.