Joao Seabra wrote an article posted at freshmeat about GCC that was enlightning:
Since my good old Pentium 166 days, I’ve liked to search for the best optimizations possible so programs can take the maximum advantage of hardware/CPU cycles. If I have a nice piece of hardware, why not run it at its full power, using every little feature? Shouldn’t we all try to get the best results from the money invested in our machines?
It has a lot of interesting info for anyone who’s trying to tailor a build to their particluar hardware. I like the bits about -march implying -mcpu but the myth debunking section rocks too.