The default wheel size for the Porsche 996 Carrera 2 was 7×17 up front and 9×17 in the rear. For the pre-facelift model, the default wheel appears to have been the M396 turbo-look wheel.  For the facelift 996.2 model, this was the 10-spoke Carrera II wheel. The Carrera 4 used a different five-spoke design.

I’m aware of four optional 18″ wheels for the facelift, in 8×18 for the front, and 10×18 for the rear:

• The five-spoke Carrera wheel
• The five-spoke Turbo Look I wheel, essentially the 996.1 standard wheel in a larger diameter (for the pre-facelift car these were offered in 7.5″/10″ width). There’s also a Turbo wheel with hollow spokes that is lighter.
• The two-part Y-spoke SportClassic II wheel
• The two-part 10-spoke SportDesign wheel

All of these seem to have been made by BBS. My favourite is the Carrera wheel, which also appears to be the lightest of the bunch according to the information at 944 racing. While I’m generally a fan of Y-spoke wheels, I find the SportClassic II a bit busy.