personally, i would try taking some of the load off the psu, and trying that first. with a dead fan on the psu it could be the simple fact that its not running efficient.

remove anything that you wont need.... sound card, cd drives, spare harddrives. try and see if that helps with the restarting.

most harddrive manufactures have a "disk testing program" to run stress tests on the drive to see if its functioning properly. i know that western digital do one, as i had to test one of mine, but as ive not had to use others, im not 100% sure.

that sounds like a powersupply issue, but its very hard to tell.

does the HDD make clicking noises, slow to respond. try running ATTO disk benchmark on it, and if its falling below normal speeds for a drive similar.....

put linux on it! suse10.2 if you like to learn and a good challenge.

or put ubuntu on it if your after an easy to use op system.

or run folding@home on it...... 

if your just for playing games, i would chose the e6700, as if you look at any 3dbenchmark the quad core's really arent helping with performance. save the money then for a better graphics card, or even get watercooling and overclock that little more! 

if your encoding, or converting files, then yes i would suggest get the quad, as its far faster for tasks like that. 

the price is extermely tempting, i know, as ive been looking myself, but im more of a gamer, so i stuck with my e6600..... im saving the money to get myself vapo for it, and then will hopefully overclock like a nutter.

im sure the latest drivers from nvidia have mentioned something that they finally enabled "turbocache" for the 88xx series.....

so they might have some tehnology in place that helps with caching...