Depending on the model of your ram, in the worst case, you may have to remove the front fan entirely.
Naked ram has a height of 31mm. The NH-D15's front fan will sit comfortably on top of the first stick.
Using memory with heatspreaders will force the front fan to be raised for each millimeter over the standard.
@richiestang_78
Too soon to say that without considering what the OP's ram might be.