have a read of this-
if you're gonna stick wider tyres on the 16" rims, you'll need to get a lower aspect ratio(aspect of 65 means sidewall height is 65% of tyre width, or something like that). i had 15" rims with standard tyres of 205/65 R15, so i upgraded to 225/60 R15.. more width + less sidewall = same diameter.
find out the width of your rim, and the overall diameter of your current tire. look at various tyres and see what widths you can get that will fit that rim, and that come out with the same (+- 3%) diameter.
lastly, fatter tyres dont always help grip but oh, don't they look hot!