Let me see if I can get Ubuntu testing and support added seeing as there are enough people using Ubuntu.
1. Turning off [PCI remap] feature does not make things better nor worse (except for enforcing change of MTRR settings accordingly);
2. Removinng agp=off does not change anything.
3. List of demos crashing my system: domain, double_precision_simple_matmult_d, image_filter_d, indexof, indexof_d, mandelbrot, prefix_sum, simple_matmult.
Please note that double_precision_optimized_matmult_d works correctly while double_precision_simple_matmult_d crashes.
4. Demos on list always crash my system, demos not on this list do not. I've not tested it very extensively, however - repeated few picked crashing programs (just to see yet another freeze) and run one or two non-crashing programs in a loop (plus repeated few other randomly picked to see them working again).
One more suggestion: adding a 64-bit box with >4GB RAM would be also nice - as RAM is so cheap today - I suppose 8GB will not be so uncommon among number crunching folks.
Ok, I've got what was going on.
It was propably hardware fault. I've returned that card back to the shop and bought 4850 instead. It is not officially supported but it works at least for me
Sorry for asserting false alerts.