That's probably fake.
The true difference must be +252% https://www.techpowerup.com/gpu-specs/radeon-hd-2900-xt.c192
Interesting. Though I think I actually had a HIS Radeon X1950 PRO/XT IceQ3 when I was thinking of the AGP / Oblivion playing card as the 2900 XT didn't come out until 2007, and the AGP card I was using was in either late 2005 or early 2006.
I still have a 2900 XT listed under my sold items on eBay though, so must've upgraded to that card later on.