A group called the Usual Suspects has produced a set of Shared Library files that purport to increase the speed of your 603 or 604 PPC Mac by somehow doing faster processing of code optimized for 601 machines. The ReadMe file says that the files are "designed for System 7.6 or higher. Use of the ObjectSupportLib 1.2 or higher will produce much faster results." Currently, they are in beta. Final versions are expected by the end of the month.
I have no information as to how effective these files really are. I haven't even tried them myself yet.
Update: Based on some further research with these files and some email I have received, I am removing the link to these files. I have reason to suspect these files may be a hoax.
Several reports now suggest that these files are little more than Motorola's MathLib file in disguise (both the 604 and 603 files are essentially identical). As a result, they may indeed speed up some benchmarks, in the same way that Motorola's file would.