Hi frpcastro, if I may share my own experience using a Leadtek gtx 680 and seasonic x660 PSU. The seasonic just managed to squeeeezee through with a little bit of pressure, however you will need to take out your HDD first after disconnecting all cables before attempting to remove the DELL PSU and inserting the new one.
I am not sure about the Corsair 850 size, I presume it will be about the same dimensions - pls check carefully before buying, as not all ATX compatible have exactly the same dimension.
The GPU was slightly trickier and needed some finger gymnastics as even though the card is about 10.5 inch, the 2 6pin cables comes off on top of the card so needed to find the right space. And because the Mboard only have 1 PCI 16X slot, you will need to adapt your cables a such so that all SATA and power lines can reach their destination AROUND the card.
I am sure many xps users have the same experience and mine just to confirm that it can be done.
