Take a look on your driver's door and write down the original tire size that came on the car. now go into www.tirerack.com and find that size tire in a Goodyear Eagle RS-series. Write down all the tire specs, especially the treadwidth (if any is listed) and the outside diameter. Now look at your desired 20" tires and find one that has VERY close to the same outside diameter as the originals - within +/- 0.3 inch. This will prevent engine controls and speedo errors. Treadwidth, rim width, offset, and section width determine if you will have rubbing problems in hard turns - too wide a tread is what rubs in fenderwells.
If your car came with 16" wheels and tires with a 27.2" outside diameter you could run a 27" wheel (no tire) and still keep all the controls as built. The ride would be a bit choppy...........but you get the idea - as long as you keep the tire outside diameter and treadwidth close to stock you'll never need lift kits.
The 20's WILL give you a harsher ride because there will be less air/rubber between the wheel and road.