[kosmiq]

Your PSU might be a problem if it is of bad quality, but HDD's dying sounds a little strange to me.
Any good PSU on 400w would easily be able to handle your system without any problems. 400w can take you very far and minding the fact that very few, even very new PC's with very new hardware, won't use all of the 400w avaible. What can be a problem is the maximum combined wattage on the +5v, +3.3v, -5v, -3.3v part of the PSU. But generally that won't be a problem either. Any 500w PSU of a good brand can handle 2x6800GT (SLI), 2gb RAM, AMD 64 X2 4000+, several HDD's and a few optical drives without problems.

About seagate I have only good experiences with them and I personally have 5 Seagate drives right now. 1 of them I have had for 5 years now (30gb IDE), 2 of them for 2 years (160gb SATA) and 1 for about a year (200gb IDE).
I also have 1 maxtor drive 200gb IDE. None of the HDD's has ever failed me or given me any problems whatsoever. Many of my friends also uses seagate drives in their PC's and have no problems.
Cheap Seagates? Seagate are more expensive than Maxtor or WD whenever you purchase a seagate 7200.7 or 7200.8 Barracuda drives and a lot more reliable. Lets not forget the fact that they are also making less noise than most WD drives.

If you want to try things out try another brand of HDD and see if the problem still comes. If it does I say something else is wrong and would recommend you to change PSU.