Best option is to do a quick search for serial RAM. Few days back something pretty similar was discussed. Look at the following post: http://www.picbasic.co.uk/forum/showthread.php?t=12766

With an FRAM IC or a serial SRAM IC you can have your cake and eat it too. Need a PIC with HW SPI (4 lines), which the pic18f2620 does, and you are in business. At 14,400_bytes-sec input rate (115200bps), you could be buffering the data into RAM at a rate close to 625_bytes-sec (yes this is 5Mbit-sec). If this is not enough I do not know what is. Key in all this is the SPI bus capability in both the memory and the PIC.

FRAM is specific to RAMTRON, but is non-volatile (a bit expensive). Serial SRAM can be found from OnSemi, Microchip, and others...