Computer freezing whilst playing games after installation of new graphics cards

I seem to be having similar problems to the people in these threads:

(I cant place links so please add two Ws to the beginning of each web address)
w.techspot.com/vb/topic158129.html
w.techspot.com/vb/topic163256.html

In that my computer will almost randomly crash whilst playing games (the monitor will fail to receive signal from the PC, the most recent audio will become stuck in a loop the computer will remain on until I manually switch it off by holding the power button down.) This is a new error that has occurred on my computer having had both graphics cards (Nvidia geforce 9800GT) replaced due to both suffering from hardware problems as confirmed by a computer repair company (Kehtron computers).

Both cards were replaced with NVIDIA GeForce GTS 450 Which are quite large and the fan of the top card is partially obscured by my wireless internet card (linksys). The problem seems to occur anywhere from 5 minutes into a game(such as Empire:Total War) to not at all. The problem seems to coincide with specifically online games though I find this to be questionable as most if not all games I play are from Steam.

I don't believe the problem to be temperature related as I have been using the msi afterburner to monitor the temperature and fan speed to keep the temperature below 60 degrees centigrade(though I cannot confirm this whilst playing the game at the time of the crash, I routinely tab out to check the temperature).

My Specs are very similar to the second poster above

Dell XPS
Windows Vista-64 bit
Intel Core 2 Quad CPU 2.66 GHz
2 linked NVIDIA GeForce GTS 450 graphics cards

I'm aware of the peculiarity of the problem and would appreciate any help that can be provided. There don't appear to have been any minidump files for the most recent crash but I will keep an eye out if any appear.
 
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