I recently committed a fix for hangs on rs780/880 chips in certain cases:
ahhh.. this sucks. I hope the devs give the 780G/785G/790GX chipset some love. We've got several AMD Quad-Core systems using this chipset still, and were planning on upgrading to Ubuntu 13.04 when it came out, knowing full well, we would lose Catalyst support. We always thought the OSS radeon driver was an option, but it sounds like this might be a problem.
By the way, I do want to add that I built an AMD Dual Core system for my kids using a 790GX motherboard, and with the catalyst driver, it was fast enough for them to play Elder Scrolls: Oblivion and Dawn of War 2 in Wine. So these chipsets are not bad if you stick with slightly older games (2005/2006 timeframe), which my kids enjoy. Its really a shame that AMD catalyst driver no longer supports the latest X.org.
Just to add a datapoint; I have a 790gx northbridge with the 3300 integrated graphics. I use the open radeon driver, and its been stable for me. WoP is playable with 30-50 FPS, and I was able to play the penumbra games. This machine is about 4 years old and only has a dual core 5050e low power cpu. I wouldnt tout it as a gaming rig, but its far from a disaster;)
I bet Ubuntu 9.04 would fly on that thing using catalyst.