High-speed RS232 logging
According to my reading of the datasheet, an Atmel ATmega running at 20MHz (the max) can run its UART at up to 2222222 baud rate (~2.2Mbps), more than double the speed you need.

(BAUD = Fosc / (8 * UBBRn + 1)) 
   with Fosc = 20000000 (20MHz) 
   and UBBRn = 0 (the minimum)


So the UART reading part should be no problem. The only question now becomes the writing to the SD card. You can do that over SPI, which is separate from the UART, so you can run them both in parallel with little overhead. According to this site, when running an ATmega at 8MHz (thus 4MHz SPI clock), you can write to a SD card at up to 140kB/sec. You only need ~128kB/sec, and ATmega's can run up to 20MHz (thus 10MHz SPI clock), so there shouldn't be a problem there.