Ubuntu Memory type ddr2 ddr3

$ sudo dmidecode --type 17

$ sudo lshw -short -C memory
 
From [1]

DDR1: 2.5 V, 133–200 MHz
DDR2: 200–400 MHz
DDR3: 1.5 V, 400–800 MHz (up to 1400 MHz for super-high-end)
DDR4: 1.2 V, 2133–4266 MHz

For example: if shows up in lshw -short -C memory as simply 16GiB DIMM Synchronous 2133 MHz (0.5 ns). The MHz rating indicates that it's almost certainly DDR4.

References:
[1] memory types http://en.wikipedia.org/wiki/Synchronous_dynamic_random-access_memory#Generations_of_SDRAM
[2]