recommend good DDR4 RAM
For the $2,500 option with overclocking, you recommend DRR4-2133 memory. For $10-$20 more you can get DDR4-2400 memory.
For a gaming rig however this has been shown to make essentially no difference in real world performance, which is why these upgrades are not done.
Capacity is much more important than speed however if you would like to make that change for a specific purpose or just because you want to, it is possible through PCPartPicker.