Have you tried running it with the ATI overdrive off?
I have a Dell Dimension 8400 with an ATI Radeon X850XT card (and two Gb RAM), but in addition to upgrading the PSU, I also installed a second card fan.
Have had no problems - but I've never unlocked the ATI overclock function. And I have no cats.
[Edited to note that EP4 installed and working fine]