Alex Granovsky presented the results of the PC GAMESS (version of Gamess US designed to work better on x86 architecture) benchmarks on new AMD Opteron and Intel Core2 processors. You can find them here.
The most important observations are:
- Intel CPUs are much faster,
- but AMDs scale better with number of cores.
BTW. You can find other interesting benchmark in the Performance section of the PC GAMESS website.