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They used the same ones in all the XPS 8300/8500/8700. I remember having a conversation with the Dell reps :)
I don't know why you are so worried about the power supply failing. I have a 7870 on another XPS 8500 thats the same age on my one which came...
I have the XPS 8500. It is the pretty much the same apart from the motherboard and processor. I've been running a GTX 660 ti on the stock 460w PSU...
Sounds good. Glad your happy with it! :)