do you have an 80 wire IDE cable? if not, that could be slowing your hard drive down.
also, is S.M.A.R.T enabled?
disable it if it is.
I don't think it's the PSU, otherwse it would just be crashing randomly.
test your memory with memtest, to rule out bad RAM
also, you said you can swap the video card between a TNT2 and a Radeon 9800. that would be very strange unless the board is an AGP 2x/4x board only. if it supported 8x, it would not support 2x and therefore not be able to run the TNT2
what board did you say you had again?