Graphics cards keep frying (I think)

Recently I upgraded the cooling system on my radeon x700 pro PCI-E card from the stock cooler to an arctic cooling vga silencer. The first time, it worked great and I saw a definite boost in performance with gaming (HL2, FarCry, Silent Hill 3/4, etc.). I left my computer on (idle with no programs running) over night when I went home from college, and when I got back to school, my computer was off and would not start up as long as my card was in the PCI-e slot... I got a new card and re-attached the arctic cooling fan and everything worked fine still, until the exact same scenarion repeated itself. I know that I put plenty of thermal paste on and had all the latest drivers...it only broke down when I left it alone for ~24 hours.
Any advice would be GREATLY appreciated. I'm at a loss here! I've also tried plugging in the stock cooling fans, but to no avail. (didn't work).

Dell Dimensions 4700
p4, 2.8 GHz, 800 FSB
1024Mb DDR2
40 Gb SATA HDD
160 GB SATA HDD
16x DVD +/-r
Windows XP SP 2

 

You're only supposed to put a thin coating of paste on. How many watts does your PSU supply?

 

The PSU is 350W.

 

Well with the next card, i would leave the stock heatsink and fan on.... I mean if the card is not overheating in the first place, why mess with it?