Consensus in the Impala scene is that the best looking wheel size on '00-05 Impalas is 18s, but you can ride 20s without modification. Your ride quality is going to suck with 20s, and you're going to have to watch how you drive, because the low-profile tires do not offer much protection to the rim. So, avoid areas with lots of potholes or rough roads. 22s will fit but you will have to roll your fenders, and do some other small modifications. At this point though the tires are basically going to be rubber bands covering the rims. I have also seen a 2000-2005 Impala riding on 24s, but at that point the car looks pretty ridiculous.
If forgot about this... Running 22s or higher will put a lot of stress on the transaxle, thus reducing the life and reliability of the transmission. I figure that is something to really take into consideration when choosing wheels.