Use the article linked in my previous post. Yes, 850 is a good number, perhaps even for Crossfire (depending on the cards you install). Thermaltake has a PSU calculator webpage that you can use to get a general idea of the necessary wattage you need. I normally take what it says and add about 100~150 watts extra to account for degradation (wattage output degrades overtime).
Now, whether it's the PSU or the GPU, I am not entirely 100% sure on that. Your mention of the motherboard also has me curious. The problem is, all of them are suspect, and there's really no way to isolate or test either without swapping hardware or isolating in another reliable environment.
I think it's best to start first with the item that's actually crying foul: the GPU itself. Since commonly the crashing is because of the ATI driver alerting the system of a GPU problem, I reckon it's best to start there first, then approach the motherboard since it has not listed the GPU as supported (it looks like a very limited and old list though), then follow up with the PSU.
That, or you could skip over all this and approach a local PC shop to have their techs look at it. They very well may have the right environment, resources and tools to properly approach this. Mention to them the details provided on this thread so far (especially the BSODs being straight from the ATI driver).