can the SD card be used as System Memory?
No, and no. An SD card is far too slow to be used as system memory. It could be used as swap or cache (which is what Windows can use certain types of memory for). OS X doesn’t have a very accessible method to designate it as cache or swap, though it’s possible with a little bit of hacking. I don’t think that it would buy you much, based on the strategy OS X uses for caching and prebinding.
