the stuff posted by the previous poster is quite accurate, but let's not lose sight of the fact that this is DATA SUFFICIENCY - and that, therefore, a lot of the calculations in that poster's workup are unnecessary.
what follows is the bare minimum of work required to establish the answer to this problem. even if you have no trouble understanding the answer, pay attention to the STOPPING POINT at which you can conclude that each statement is sufficient; it comes earlier than many of you might have thought.

smclean23 wrote: (1) 2 M-type memory units will increase the computers base memory capacity by 300 percent

as the previous poster has recognized, you don't have to treat this as a percentage change problem at all.
just realize that 2 x 3mb, or 6mb, is 300% of the original capacity.
STOP - no need to calculate this value
sufficient
alternatively, if you'd rather treat this problem as a traditional % increase problem, realize that base memory + 6mb is 100% + 300%, or 400%, of base memory itself. letting x stand for base memory, we have x + 6 = 4x.
STOP - no need to solve this equation
sufficient

smclean23 wrote:(2) The memory capacity of the computer after 2 M-type memory units are added to the base memory capacity is 1.6 times the memory capacity of the computer after 1 M-type memory unit is added to the base memory capacity.

x + 6 = 1.6(x + 3)
STOP - no need to solve this equation, especially with those annoying decimals ruining the fun for everyone
sufficient

answer = d

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by the way, i have a hunch that the gmat would be likely to replace "1.6 times" with something requiring an additional step or two, such as "60% greater than" or "8:5 ratio". the current version of statement 2 is just too perfect - the equation practically writes itself.