Short answer: G450-G550 Matroxes are best for 2D and for multi-head.
Though for some weird reason my G450 didn't work in the latest
Ubuntu. :-( (I only installed it because my ATI Radeon R100 stopped
working due to a bug in the radeon driver!) I ended up getting a used
Radeon 9200, which works great. Haven't tried to go dual with it yet
It seems to be easier to get working cards if you don't care about DVI
output, since there seems to be a small window between when manufacturers
started supporting DVI and when they stopped providing open specs.