-O and other optimization flags

[zhou13@xxxxxxxxxxxx]

Hello GCC-Help list:

I am wondering if i have used -O, -O2 or -O3, do i still benifit from 
flags such as -march -fmpmath -ffast-math -mmx -sse -sse2 -3dnow?

I am optimizing a video codec and i see barely any performance 
difference whether i use just -O2 or "-ffast-math -march=athlon-xp 
-mmmx -msse -msse2 -m3dnow -mfpmath=sse,387 -O3".

The codec is at http://www.sourceforge.net/projects/openavs/. 
Currently, it requires a 3Ghz or better CPU to get a resonable 
framerate.   I would like the codec to be useful even on 586 (  1Ghz or 
so ). Any ideas?

Regards
Lionel