No. But you can swap the contents of RAM with SD in a virtual memory scheme. Just don't expect anywhere near the speed out of your SD that your RAM (it's static RAM so also known as SRAM rather than the dynamic RAM, DRAM, in your PC) runs at. If you don't do much swapping it's not too bad.
You could get a Mega 2560 and one of these for overkill:
http://ruggedcircuits.com/html/quadram.html
And the Teensy++ chip has 8k bytes of SRAM, 128k Flash (program space) and 4k EEPROM; all 4x the UNO. Note however that the Teensy is a compatible board, there are some differences though you can use the Arduino IDE.
http://www.pjrc.com/teensy/index.html
They are small and with pins they plug right into a breadboard.