I don't know what else to suggest. It seems that what ever the problem is, it may be the AGP (Which doesn't sound quite right, since the GF2 card worked), or the GF4 is faulty. (Please don't say that the GF4 works OK with windows, as there are big differences as to how both OS's handle their hardware. It will (Seem to) work ok in one, and not the other).
I know it's like
, but there must be some solution for this problem.