get the cheaper one as they don't really have performance advantage between the 2. 9800gtx however has physX.
I think AZN is right, according to this :
http://www.overclockersclub.com/reviews/powercolor_hd4850/
The 4850 and 9800GTX seem to trade blows pretty evenly.
That review is a little old and uses a 9800gtx not the + version The 4850 has gotten better since then.
I believe the 4850 is now = to a 9800gtx+/gts250
just be sure not to get the gen 1 4850 with the single slot cooler... they're hot and noisy...
They are hot because they aren't noisy!
If you get them to run cooler by increasing the fan speed they are noisy but cool.