My tool chain currently,
Slic3r for converting .stl files to .gcode
(Sfact/Skeinforge give more control but takes more time to master. Still I'm thinking of getting back into it.)
Pronterface/Printrun for controlling the printer.
SD cards are nice but not necessary.
If I'm running a new part or testing, I run it from the computer.
If I'm happy with the print and want more of the same, I move the Gcode to an SD card and print from there and use the computer to do stuff on another printer.