For shear gpu horsepower, you will more then double your potential.
9800 Pro is 8 pipes at 380 mhz
X850XT is 16 pipes at 500(?) mhz
Presumably, if the X850XT was the same clockspeed as the 9800 Pro, it would be twice as fast, but it's also running ~40% faster clockrate so the boost in performance where you are completely gpu limited should be like 2.5 - 3x.
It kinda stinks that gpu reviews haven't been showing 9800 Pro's in their charts for a long time. A lot of people still have a 9800 Pro. One good way to gauge 9800 Pro performance would be to look at the difference between a 6600GT and a X850XT and then add 10 - 20% (cuz iirc the 9800 Pro was about 10 - 20% behind a 6600GT a year ago when they still had 9800 Pro's on the chart).
EDIT: From toms last agp vga charts:
3Dmark05 at default:
9800 Pro - 2590
X850XT - 6098
Doom3 12x10 0aa/8af
9800 Pro - 32.6
X850XT - 81.4
HL2 16x12 4aa/8af
9800XT (no pro score) - 31.6
X850XT - 69.4
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