pietklerckx :
I've got an Dell Studio XPS 435MT and upgraded it with a nVidia GTX 960 and it is working great!! All running on the stock PSU of 360W (including (9 Gb RAM, 2 HDDs, 1 SSD, 1 USB 3.0 Controller).
The PC came with a Radeon 4850 but the GPU died after a couple of years and I replaced it with a GTX 550 TI which was fine at the time, but now with an GTX 960 I can play today's games with decent grapics and frame rates. I am very happy and hope others can use this information.
I have the Dell Studio XPS 435T, which is a bit larger than the 435MT, and appears to have a larger PSU, as well. Mine is 475W and has the i7 940 cpu with 6GB RAM, 1 SSD and currently is running a GTX 650 Ti 1GB video card. However, after searching Dell's community website, they do recommend a 2 GB PNY GTX 960 as an upgrade, so since I wanted more RAM, and the power draw is only 10 watts more than the GTX 650 Ti, I ordered an MSI GTX 960 4GB card. Hope it works okay. I also updated my BIOS to what is current - just in case.
I plan to build a new system, as my Dell is getting a bit old at 6 years, but since I have to wait a few months, I thought this card might just be perfect for allowing my to play my favorite games at max settings in the mean time.
I can see why a lot of video card makers are coming out with GTX 950 and 960 cards - they have low power consumption like the older cards, making them power supply friendly, and that makes them a really good upgrade option for those of us with solid but older computers.