Just so you know, HDD of the sizes: 10-20-40-80 GB were not at all reliable. Those 120-160GB are the first HDD generation that "started" some reliability trend. Still, those tend to fail more than say a 320GB drive which is 2 generations more robust.
Also since Seagate bought Maxtor years ago I found out their HDD are a lot less reliable. On dozens of Seagate HDD I bought since that Maxtor was sold, (Maxtor was super-über-mega-giga-cheap btw), I have had 10 drives that did not work at all the first time I've put it in a computer. For those that worked right away, most are still running today though.
About your 750GB HDD, it was the first generation to use perpendicular magnetic recording, so it's normal that they "may" fail since the technology was not mature at all.
My guess is that you were not lucky at all buying those not mature HDDs. HDD failure can also be related to dying memory or power supply.
You can test your memory using memtest86+ (download the "Pre-Compiled Bootable ISO (.zip)" and burn it on a cd, or download the "Pre-Compiled Bootable Binary (.zip)" and install it on a floppy/USB stick) then make it boot before the HDD. Let it test at least a cycle (about 30 minutes with a core2duo). If it shows red errors then there is hope. You will need to test them one at a time to find if only one stick is dying.
Pre-Compiled Bootable ISO (.zip)
http://www.memtest.org/download/4.20/memtest86+-4.20.iso.zip
Pre-Compiled Bootable Binary (.zip)
http://www.memtest.org/download/4.20/memtest86+-4.20.zip
That is easy and fast to test the memory sticks with this method and you should do it before trying another power supply.