In case you are interested here are the before and after benchmarks for my own P4 Northwood:
Intel(R) Pentium(R) 4 CPU 1.60GHz CPU speed: 2311.52 MHz L2 cache size: 512 KB Prime95 version 22.10, RdtscTiming=1 Best time for 384K FFT length: 17.123 ms. Best time for 448K FFT length: 20.686 ms. Best time for 512K FFT length: 23.347 ms. Best time for 640K FFT length: 30.270 ms. Best time for 768K FFT length: 37.036 ms. Best time for 896K FFT length: 45.737 ms. Best time for 1024K FFT length: 49.389 ms. Best time for 1280K FFT length: 69.955 ms. Best time for 1536K FFT length: 85.890 ms. Best time for 1792K FFT length: 104.573 ms. Intel(R) Pentium(R) 4 CPU 1.60GHz CPU speed: 2311.38 MHz L2 cache size: 512 KB Prime95 version 23.1, RdtscTiming=1 Best time for 384K FFT length: 17.175 ms. Best time for 448K FFT length: 20.320 ms. Best time for 512K FFT length: 23.232 ms. Best time for 640K FFT length: 29.986 ms. Best time for 768K FFT length: 36.869 ms. Best time for 896K FFT length: 44.585 ms. Best time for 1024K FFT length: 48.950 ms. Best time for 1280K FFT length: 65.557 ms. Best time for 1536K FFT length: 80.381 ms. Best time for 1792K FFT length: 98.204 ms.
--- Checked by AVG anti-virus system (http://www.grisoft.com). Version: 6.0.449 / Virus Database: 251 - Release Date: 1/27/2003
