For example, the voltage on an analog pin. I'd like it to be able to log in millisecond time (or nano second, considering how fast one can read/write with the ports with port registers), and then save it to an SD card.
Whoa there, let's make sure we're on the same page. Millisecond log times are entirely doable with an SD card, nanoseconds are entirely impossible. On a good day SD cards can write a few hundred kB per second (my experience...others on this forum are more expert and can probably quote more concrete numbers).
I'm thinking the best way to do it is to save the data to the arduino's internal memory, for the sake of speed, and once that is filled, write it to the SD card. However, I think the limit is the size of the arduino's internal memory; how much data can an UNO really handle?
It definitely makes sense to write data in 512-byte chunks, and using interrupts to log the data to 1 buffer while writing a 2nd buffer to SD is a good approach. Setting aside two 512-byte buffers uses up half of the UNO's 2kB of RAM. So it's doable, but leaving some room for stack space means you only have about 500-700 bytes of RAM left to play with. That might be enough, depending on your application.
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The Gadget Shield: accelerometer, RGB LED, IR transmit/receive, speaker, microphone, light sensor, potentiometer, pushbuttons