While the others are giving you good honest answers, they may not have Windows XP Pro like you indicate.
Since you have Windows XP Pro there is absolutely no reason why you shouldn't try DX9a as soon as possible. Windows XP pro allows you to set restore points before you install anything, and that includes DX9a. Then if you don't like it, you can just hit your restore point, and you'll be rid of DX9a.
I use DX9a with Windows XP Pro with no problems at all, and I did notice a frame rate increase.
Ranger Bob