At 1920 x 1080 using your new build, it would be appropriate to use a graphics system of about the power of the 5870.
Exactly what that should be is unclear at this point in time.
There are going to be some new and presumably better options arriving in time for the Christmas buying season.
There is talk of new ATI tweaks, and new Nvidia cards . It might pay to wait. Also, the GTX260 is going to put some marketing pressure on ATI so we might see some price adjustments.
My suggestion is to build, using the GTX250 to start with, and see how it goes. No doubt, you will want to do better, but getting some gaming in with the new build will clarify just how much better you need to go.
When you upgrade, it should be by a couple of performance tiers, otherwise you will be disappointed.
A 5870 may be right. Or even a 5970.
Or a GTX480,
Or SLI GTX260(they seem to scale very well)
or... whatever comes next.