this issue is VERY un-documented formally. but i will try my best to show you
I looked at the link you provided and I see some potential issues with your original claims. For example, note this statement from the Wikipedia article (the Apple TV GPU is an early VP1-era product):
time to point out a few more things.
1. VP1 cannot playback BR spec (VDPAU) - which doesnt matter, as the files will already be in a digital format.
2. we assume that the files are in h264 format (not WMV, or VC-1 etc)
3. the movies are on the hard drive.
from a practical sense of things, this
user states that it seems to play fine (no specifics unfortunately). thus far i have been unable to find any reviews comparing a singular file

i shall keep searching.
more:
this guy states that the 7300 significantly reduces the load on the CPU. this article
here backs up those claims (look at the "H.264 1080p Decoding" image) - CPU usage of 80.8% is reduced to 22.1% using the 7300.
This last statement leads me to believe that PureVideo HD is only available on the GeForce 7900 and later. In any case, can you point to any system that uses a GeForce 7300 Go that can decode 1080p30 H.264 video? I'm not talking by interpretation of specs, I mean as a hands-on example or documented report.
wikipedia isnt a very credible site

Nvidias very own website states that the
GeForce Go 7300 is part of the PureVideo HD "specification". now, VP1 doesn't completely offload all computations to the GPU, but it can reduce the CPU utilisation by 40% (
link), this post
here backs that up with a real world comparison - though the CPU is roughly 30% more powerful (keep in mind that the 7600 only has a faster clock then the 7300 (same architecture), that post uses the desktop version though of course).
^ - Which is why you need to swap out the wifi card with a crystalHD (and switch OS) if you want the apple TV to decode HD media properly.
yeh
that card will most definitely give you 100% GPGPU acceleration. seems good, but not viable for 90% of users. its $25, so pretty darn cheap!
oh and for the record: the

TV maxes out at about 44°C, well below the TjMax of the CPU (i cant speak for other components though).