I have a nVIDIA GeForce 7300 and it never had stability problems with Ubuntu (neither with the Ubuntu package nor with the latest ones downloaded from the nVIDIA site). Which drivers did Sabayon installed? I don't know, I haven't have the time to find out and now I'm kind of reluctant to boot on Sabayon again.
About OpenSUSE, I have no idea. That's what they recommended me. I have tried several distros this far. I had no problems with my old laptop (well I did with Fedora, but not with Scientific Linux or Ubuntu) but it got stolen.
I have Unbuntu in a 32-bit desktop at work and I'm very happy with it. The problem with the laptop is that I couldn't configure the Wifi card (even ndiswrapper wasn't working) and none of the Flash plugins was working. There were also minor issues with the sound card.
I think Sabayon is great but, unfortunately, a 64-bit laptop can still be a tricky issue. I was very naif to think that it was going to work straightforwardly. So I will have to go back to the old manual-editing configuration stuff. Anyway, if you know of any distribution that is more likely than others to work on this hardware, please, let me know, my ears are open...