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[msi1337]

 

[videopho]

First I'd check to see the new card wouldphysically fit inside your 5100. The 7900GTO is doubled slot card so it'd take up another extra pci slot next to it, besides this card is long. I do know this, my 7900gtx barely fits inside the dell gen 4 chassis.
As for the psu, you're probably okay with what you have. Dell's psu design is proprietary one. That said you have to go out and buy what works for your dell. You cannot simply pop in any atx universal psu in there and expect it to work.