Hmm, yeah, I'm definitely starting to lean towards the video card, having played around with it a bit more. I'll try disabling the nvidia drivers and seeing if I stop getting kernel panics and the likes. If that at least stabilizes the system, then I'll be fairly confident it's a video card issue. Then I'll look into swapping the video card for something that's got officially supported drivers... any recommendations? It doens't have to be top of the line. I guess anything I can borrow from someone that can sit in a PCIEX16 slot and supports hardware acceleration will do. I guess ATI is the correct answer, but I've not been on the market for video cards in a long time. I should also contact ASUS and find out if there's a known issue with that mobo + enabling a GPU + ubuntu, but I don't think they'll be of any help...

and so, the saga continues