Actually to say something positive, I have just tried Win7 for this particular reason, and in experience it was much smoother during intensive write-read tasks on the OS disk, than using XP 64. Using HD Speed it produced the same results, BTW.
My actual problem is that under XP 64 my computer freezes for moments or seconds when I do a huge data copy from one hard drive to another. With big files it could get over 110 MB/s in Total Commander, and during this copy my computer is unusable! It freezes the mouse for seconds sometimes! That's why I all started finding why is that happening. Actually I tried a simple seek rate test during these copy operations and it became 1200 ms average, with 2500-3000 ms peeks! On XP 32-bit it was about 25-35 which is normal I think for heavy file copy, but not 2-3 seconds as on XP 64!