So if you switch back to your old card do things still crash? Is anything else playing up on your PC, processes segfaulting, kernel panics, boot process sometimes failing, lots of rude messages in the dmesg logs, etc ?
The question I\'m getting at is, what has changed between when it was playing fine, and when it was crashing ? You arent overclocking your PCI or AGP slots are you, that probably wouldnt go too well with video cards that are expecting a nice clean 33 or 66MHz bus signal..
Your RAM is a little on the light side too, I would recommend trimming down your environment as much as possible if running with 256 MB. Like shutting down init.d processes that you dont need, switching to a lightweight window manager rather than Gnome or KDE with all the bells and whistles, and so on.