The problem I am finding trying to help you is that the FX 5900 is such an awful card that no one seems to have bothered benching it in 3DMark 2003, and 2001SE wouldn't give a good indicator of the 3850s superiority over the 5900.
Edit:
Okay, here is a list of FX 5900 benchmarks: Linky.
And here is a list of HD 3850 benchmarks: Linky.
The card you have is way down the list compared to the 3850.
Its still not as good as a 8800gt card though. Close But it may be overkill for your system. The processor will slow you down a ton.