> I've never seen the BTO X850XT

It can be bought for significant money :-)

https://www.google.com/search?q=661-3594

>>> there is one higher card that will work with the G5 as well,
>>> a standard PC X1950Pro AGP card.

>> Wouldn't this also be true for other cards from X1xxx series?

> In the case of the R500, I know cards not even listed can work.
> If I remember correctly, Ffrank even bragged about having an X1950XTX

I was more thinking of X1300, X1550, X1600, X1800 and X1900, which are listed as supported by MorphOS.