ASUS 8800GT in XP 64Bit

[zhouhenry]

I am a little tired of VIsta X86 system. Since the memory size is 4Gb, so I switch to XP 64Bit system. Now comes a problem. I am currently playing a video game Mount and Blade which running perfectly(over 80 Frame Per Second) in Vista System, however, since I switched to Xp64bit, it runs so bad sometimes even freezing for a second (less than 20 fps).
I believe I installed the correct drivers for MainBoard/Graphic card (both in authorized XP 64Bit drivers)
Does anyone know why?

System specification
Q6600
Gigabyte EP35 DS4
Asus Video Card EN8800 GT
Corsair Memory 4GB
Maxtor Diamond 500Gb eSATA
....