Get a 6800 for that price range. Why? its as good as an x800 BUT it has shader model 3.0. Considering it looks like you dont upgrade often, you'll want the Sm3 support, which a x800 wont give you...basically, you'll just want SM3 for next gen games, and FEAR uses it, and it is good.
You need Sm3. Trust me.
So do yourself a favor and get a 6800 (or 6800gt if you could).
And don't listen to anyone who may flame me

lol