as long as its single channel set up of ddr ram you can add similar speed sticks , size doesnt matter.
AE....
512 2700 ddr and a 256 2700 ddr will work fine in most pc's
if you mix a 512 2100 ddr and a 512 2700 ddr you will be limited to the speed of the 2100 ddr stick.
you can not mix sdram and ddr type rams though .
and if your running Duel channel you its best to have the exact same brand,type, size,and speed installed.
If you get tons of "Beeps" at boot up that ussually means the ram isnt right . (its ussually 1 long beep then alot of short beeps)