I did a heavy comparison on my machine between the 2 OS's. Here are my specs:
Athlon 64 3000+
1.5 GB of Mushkin Basic Ram
x800Pro AGP Video Card
2 Seagate 80Gb SATA Drives in RAID 0
nVidia nForce 3 250 Chipset
The OS's were both installed the same way and the drivers wereinstalled in the same order to prevent weird little quirks with the system.
Here are the comparisons:
(The First set is the XP 32bit score, the second set is x64)
Sisoft Sandra:
- Drhystone ALU - 8420 MIPS - 9935 MIPS
- Whetstone FPU - 3181 MFLOPS - 3624 MFLOPS
- Ram Bandwidth INT - 2222MB/s - 2230MB/s
- Ram Bandwidth Float - 2213MB/s - 2231MB/s
- Drive Index (HDD) - 92MB/s - 91MB/s
HD Tach:
- Random Access Time - 13ms - 13.6ms
- Read Burst Speed - 149.5MB/s - 147.3MB/s
- Read Speed Avg - 50.9MB/s - 50.3MB/s
- CPU Utilization - 27.5% - 34.6%
Super Pi:
- 1M Calculation - 49s - 50s
PCMark '05:
3DMark 2001 SE:
3DMark '05:
There you have it. The HDD access times went up, but that is the only thing. I beleive that is due to the drivers for my chipset and RAID are in BETA, and if nVidia would ever get off their arses we would see some better HDD performance. But the HDD performance scores are so little in difference, you will not notice... and when it comes to games, HDD access does not really matter. Over all, XP x64's 32bit performance is better that Windows XP 32bit

.
BTW: I originally did these tests for some people in the Planetside community, but have been posting them around when they have been asked. They were done about 3 weeks ago.