I got myself a new (used) card today, a Quadro FX 1100 AGP, quite an old card, but thought I'd try it on my pegasos. Here are the card specs:
http://www.pny.com/products/quadro/fx/1100fx.asp
As it turns out it runs PERFECT on my pegasos II, it's WAAY faster than the standard 9250 and it's the first time I actually feel the system works fine at higher resolutions (the radeon sucks at anything over 1024x768).
I haven't tried nouveau yet, but I've used it for quite some on my powerbook (nv34, the quadro is nv3x as well) so I don't expect any problems from the driver too.
Btw, I got this card from a friend for around 50e, it can be found for similar amounts on ebay. If you want a fast card on the pegasos and don't mind waiting for 3d with nouveau, do yourself a favour and get such a card :D