Here's the problem...
Anything you get in that range will be built on increasingly obsolete tech (AGP, IDE, etc.) and while it will be up to spec for TF2, you will have severely limited options for upgrading when the next game that catches your fancy maxes out your machine. While I could probably cobble together parts for recommendation on building an up-to-spec machine on that budget, I would really recommend waiting while you save for a better build.
In the meantime... how old is your install of XP? You might benefit from a format and clean slate install of XP.