X1950 IMO but never used an X850 (perhaps someone else can chime in)
I picked an x1950 pro up a few years ago for a temporary card while I was waiting for another to arrive and it was actually a pretty impressive card for what it was. It held it's own in any source game.
Quite a bit of difference, in shader intensive games.
The x1950 has roughly 2x the pure shader power then the x850 series, and a faster mem speed. ANd it also is SM3.0, which allows some better lighting effects. In bare fill rate, it does lose a little bit.