Missed this post the first time round.
I'm not too familiar with the technical specifications of the older consoles. I wouldn't be surprised if MS ordered a slightly customized variant of the GeForce 3 that did things a bit uniquely as that was how consoles were designed for the most part back then with little bits of more advanced tech than generally available.
Well, it was definitely a variant of the GeForce 3. Per Wikipedia, the GeForce 3 used the NV20 chip, the Xbox used the NV2A. Found this on an old Overclockers forum post from 2001:
GeForce 2 GTS
Core:200MHz
Memory:333MHz
Raw Fillrate:800 Mpixels / 1.6 Gtexel
--------------------------------------
GeForce 3
Core:200MHz
Memory:460MHz
Raw Fillrate:800 Mpixels / 3.2 Gtexel
--------------------------------------
NV2A
Core:250MHz
Memory:266MHz (PC2100) 64MB Total
Raw Fillrate:4.0 Gpixels / 4.0 Gtexel (8 pipelines)
An Anandtech article from the time says this:
You can take the codename to mean that the integrated graphics offers performance and features somewhere in-between the currently available NV20 (GeForce3) core and the upcoming NV25 core.
NV25 became the GeForce 4. So if I had to guess, I'd say the NV2A was just too expensive for nVidia because it was some weird, unique hybrid chip. Could definitely see that playing a role in the demise of the OG Xbox.