I had managed Cool'n'Quiet by cpufreqd.
I changed it to powernowd and it works better.
Cool'n'Quiet ON by powernowd
SURFACE GET BITS: 866.771 M/s
SURFACE PUT BITS: 1246.87 M/s
MPEG DECODING (1920x1080): 69 pic/s <== OK
VC1 DECODING (1440x1080): 110 pic/s <== OK
MIXER WEAVE (1920x1080): 2380 frames/s
MIXER BOB (1920x1080): 4219 fields/s
MIXER TEMPORAL (1920x1080): 826 fields/s
MIXER TEMPORAL_SPATIAL (1920x1080): 306 fields/s
9800GT Low Power (Core 550MHz/Memory 1800MHz)
I think the bandwidth@CnQ is enough.