Definitely a specific kind

You want your magnesium to come with calcium - the two work together for muscle contraction and relaxtion. You want to get a chelated version - something that ends in 'ate' like malate or citrate (but not carbonate as it's harder to absorb) and you should find the two minerals together in a 2:1 ratio (usually 333mg calcium / 167 mg magnesium).

They should be taken in two doses, at least - the body can only use so much at once. RDA for calcium is 1000mg and for magnesium its 500mg. I typically take one with 3 meals and save one for bedtime.

HTH
Nat