"Why not forget the large small ram and just transmit whenever a button is pressed to the PC which has a cheap large large ram ? Use a 115200 baud rate and you can get a decent 10kHz sampling rate."

Why not forget the 8051 altogether and use an A/D card in your PC? Or perhaps you could forget about the A/D card and use the joystick port instead?

Either way you could also forget the cheap large large RAM and use the cheap cheap large large large slower but still very fast hard disk RAM.