I had 19" rims for a couple of weeks until I noticed that they didn't look very fly. Trust me, don't waste your money on 19's and go straight for 21's with ultra-low profile tires... you'll never go back. I think it's really what the designers had in mind for the IS (they look SO GOOD), but they just backed out at the last minute. Also, be sure to get a dual exhaust with tips that are at least 12" in diameter... they make the car DA BOMB BABY BOMB BABY! Don't forget lots of stickers.
Go to the library and find the recent Road and Track (within the last year) or Car and Driver that did a test of 17, 18 and 19 inch wheels. Usually the 17 performed the best and sometimes the 18. The 19" never increased performance, especially considering the cost. I would go "plus one" at the most (18") or just get a 17" with another 1/2 inch of width.