the OP may be discussing agp cards here. i cant think of any other reason to be comparing such old cards. the 3850 is alot faster and supports directX 10.
To answer the question correctly.
The 3850 is almost twice as fast as the X1950PRO. and the 3850 has a special feature known as Power play meaning the GPU will run at 300MHz while running 2D applications but as soon as the GPU is taxed such as in a 3D applications, the GPU speed will jump from 300MHz to 668MHz.
Please note: The AGP version of the 3850 doesn't support Power play, so it runs at 668MHz all the time. I found this out the hard way. I own 1 3850PCI-E, 1 3850 AGP, 1 X1950PRO AGP. 1 X1950PRO PCI-E and 1 Geforce 7800GS OC AGP.
I would think the biggest difference would obviously be the performance but keep in mind the 3850 isn't supported by ATI and AGP will be dead soon enough.
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