If I take an SAT Subject Test, will it bump up my SAT score? My SAT score isn't very high but I think I can do really well on the Physics Subject Test. I'm interested in engineering and computers.
No, your SAT Subject Test scores do not directly affect your score on the regular SAT.
That being said, scoring well on SAT Subject Tests is one way to offset low SAT scores (and can also demonstrate your knowledge and interested in a subject). Not all colleges require SAT subject tests, but some schools allow you to submit extra tests as evidence of your interest in a subject (like your interest in engineering). In other cases, some "test-flexible" schools may allow you to submit SAT Subject Test scores in place of SAT or ACT scores.
However, if you're worried that your low SAT score is going to hurt you in the admissions process, you'd probably be better off studying hard and taking the SAT again, rather than just taking an SAT Subject Test.