umm, yeah.. forgot about that.. well, i'm not really sure if you can / how do you calculate it yourself, so i generally check it on a couple of webistes (to make sure they both give the same numbers) for the recommended rim width for a given tyre width.
checked it for 145 (your current ones) and the recommended size (rim width) is from 4" to 5.5".. i'm guessing your rim must be 4" or 4.5".
for 165, the recommended rim width is from 4.5" to 6".. kindly check what your current rim width is... (to see if we can use the same one or need to buy a new one)..
but even if your rim is 4" (less than the recommended rim size for 165), i'd still say try putting the 165 on it, see how it looks, if the bulge is too much, buy new rims, otherwise use the same ones.
Kindly check your rim size,