dell inspiron 3421

You may use up to 8 GB (two x 4 GB modules) of DDR3 or DDR3L RAM up to 1600 MHz. Here is Crucial RAM that is guaranteed compatible with your laptop.

Whenever touching components or working inside a computer, wear a grounded wrist strap, also called anti-static wrist strap, ESD wrist strap, or ground bracelet. It's a cheap and sensible precaution. Rest the laptop on an anti-static mat or at least a reasonable alternative such as corrugated cardboard. (Repeatedly touching a metal case part is not sufficient mitigation with modern components.)

Até 32GB e qualquer frequência serve mas notebooks básicos como as séries 3000  e 5000 dificilmente passam de 2400 ou 2666MHz pois são notebook para coisas simples como office e Internet.  Se quiser frequência maior nas memórias somente encontra nos modelos gamers ou precisions