100ns and 120ns are access times, which is the inverse of speed. Lower access time means higher speed (faster operation).
Typically, you want replacements to be the same speed or faster; which means the same access time or LOWER.
That said, many things were designed and/or built with significant margin, and will work with slower devices. So, I'd say: there's a good chance a 120ns device will work fine... but you'll be taking your chances as there's no guarantee.
Quick answer depends on the board and the ram chip.
Safest answer just replace it with the same speed.
Longer jibberish answer
If your board was designed to use 120 nanosecond ram chips. Putting a 120 where a bad 100 is isn't going to effect you much.
If your board was designed to use 100 nanosecond chips. You will be over driving your 120. Not a real problem if your using nec brand chips but you do add an element that might add a possible unstabliness in your game.