Yeah, 1.6GHz ist the lowest entry to G5 and a G5 >2GHz is the better choice. But even a 1.6GHz makes sense (in case you can get a cheap one), if you need to use additional cards, like SCSI or more USB-ports.....
I've G5 with 1.8/2.0/2.3GHz and for most applications can't see any remarkable difference in speed during everdays work. Only when useing Wayfarer or OWB (Youtube) or coding videos, I notice a real difference.