chanwei21 :
what are the difference between DDR4-2800 vs DDR3-2800?
There are a large number of differences between DDR3 and DDR4.
The highest native data-rate for DDR3 that is produced by any manufacturer is DDR3-2133. Anything above this is overclocked.
The highest native data-rate for DDR4 that is currently produced by any manufacturer is DDR4-2400, this is expected to exceed DDR4-3200 in the future. This means that current DDR4-2800 modules are based on DDR4-2133 or DDR4-2400 chips. In the future they may use native DDR4-2800 chips when they become available.