I dunno if RAID would help, but FlaskMPEG certainly isn't reading or Writing to your Harddrive THAT fast so it needs a RAID set up

I've tried Tbird 1.2 encoding on FalskMPEg and it was okay...
Next i tried P4 1.7, and it was actually better. Remember, i'm not using Intel's Optimized version. What i use is the Flask 0.594 Miha Vanilla version.
Set up for P4 system:
Bitrate+Codec: 4000 Kbps Low Motion
Audio 128 Kbps/48,000Hz
iDCT: IEEE reference (Slowest speed, highest quality)
Average i got for Athlon was 12Fps, and that was on MMX iDCT (Highest speed, worst quality). And it was 17-18 Fps on the P4 1.7 with the above setting. I guess the bandwidth of the RDRAM makes all the difference. Just my 2 cents