first bit<byte<kilobyte<megabyte<gigabyte<terabyte<petrabyte
the next after mb is gb not tb
and the memory size doesnt matter as much as actual card performance.
also when you SLI cards you dont get twice the memory per se. and you dont get twice the performance. but 2 cards in SLI will beat 1 card with twice the memory.
be more specific though what cards?