I was on newegg and how people comment out the products they just bought. Here's one comment on Amd X2 4400 said:
Windows does not properly handle all of the features of this chip yet. See microsoft tech article 896256. This resulted in reduced performance which was correctable by installing the hotfix which was provided by Microsoft following significant prodding.
There were some concerns about dual core compatibility with XP Home since it was only supposed to support 1 processor. However, users with home have had no problems with 2 logical cores via Hyperthreading and generally dual core processors are also supported with no issues. It's probably best to use XP Pro to avoid possible problems anyways though.
As well, AMD X2 processors have been known to have occasional problems with XP and 2003. The KB896256 article that you listed described such problems. I'm not sure if the hotfix is available for download since the article asks you to phone Microsoft to request it. It may appear in Windows Update though if it detects an X2.
You should also install the "AMD Athlon™ 64 X2 Dual Core Processor Driver for Windows XP and Windows Server 2003 Version (exe) 126.96.36.199" that is available at the AMD website. I'm not sure if this is the same or similar to the problem Microsoft was addressing, but this will allow the OS to control the performance of the CPU via clock speed, voltage, etc. http://www.amd.com/us-en/Processors/TechnicalResources/0,,30_182_871_13118,00.html