Thank you for your post.
Windows XP was released after FileMaker Pro 5.5, and FileMaker Pro 6 was the first version certified with Windows XP. Even though I do not see any problems reported with FileMaker Pro 5 and Windows XP, I would use caution when using FileMaker Pro 5 with Windows XP.
We used FMP 5.5 with (I think FM server 5 as the host) on Windows XP for many years with no problem. I'd recommend upgrading to 10, FMP 5's days are numbered and FMP 10 offers many many improvements over FMP 5.
My concerns were "tilted" more towards the "dual core" portion of the question.....