GTA4流畅不流畅，问题不在CPU？

hkckk

本身的：
1.02补丁，8500GT，5400+ 2.8G（没有超频），开800*600还是卡
Statistics
Average FPS: 24.28
Duration: 37.12 sec
CPU Usage: 89%
System memory usage: 85%
Video memory usage: 99%
Graphics Settings
Video Mode: 800 x 600 (60 Hz)
Texture Quality: Low
Render Quality: Low
Reflection Resolution: Low
Water Quality: Low
Shadow Quality: Low
View Distance: 1
Detail Distance: 1
Definition: Off
VSync: On
Hardware
Microsoft Windows XP Professional
Service Pack 3
Video Adapter: NVIDIA GeForce 8500 GT
Video Driver version: 181.20
Audio Adapter: Realtek HD Audio output
AMD Athlon(tm) 64 X2 Dual Core Processor 5400+

但换了新的以后。。
Statistics
Average FPS: 42.21
Duration: 37.13 sec
CPU Usage: 90%
System memory usage: 47%
Video memory usage: 71%
Graphics Settings
Video Mode: 1024 x 768 (60 Hz)
Texture Quality: High
Render Quality: High
Reflection Resolution: High
Water Quality: High
Shadow Quality: High
View Distance: 1
Detail Distance: 1
Definition: Off
VSync: On
Hardware
Microsoft Windows XP Professional
Service Pack 3
Video Adapter: NVIDIA GeForce 9600 GT
Video Driver version: 181.20
Audio Adapter: Realtek HD Audio output
AMD Athlon(tm) 64 X2 Dual Core Processor 5400+

不懂贴图。。
相差咋那么大了，难道CPU并不是最重要的瓶颈？