inching up on a 99 4.6 hse

Hi, I am thinking about getting bigger rims on my 1999 Rover 4.6 HSE, it already has 18 inch rims, and I am thinking about 20 inch rims. A few people I talked to said there might be complications with inching up. Does anyone know what the problems will be, and will low profile tires solve this problem? Any help would be greatly accepted, as I am in dire need of new tires, the rubber on them is splitting and cracking and I have already had to put a silicone solution inside one of the tires to prevent it from leaking. Thanks!