RE: XPS 15 fan, on/off/on/off, gahhh!!!
I can't remember exactly which i5 it has, but my best guess is the Intel Core i5 (2nd Gen) 2430M / 2.4 GHz with turbo boost up to 3 Ghz (found the model on CNET). I think it's the only i5 option on this model,. It also has the FHD 1080p display which might add some stress and heating on the GPU.
Yes, the palmrest heated a lot, but I swapped to an SSD and put the original HDD in the DVD-ROM, so that isn't such a big issue anymore. A guy I went to college with, had an XPS model older than mine and his palmrest completely changed colour from overheating in less than 2 years.
The XPS did its job very well and it still does for who I gave it to because overall it's robust. But if you want a laptop that's silent, my adivice is to invest in something else. If you can't keep the fans low by setting maximum processor state in power management with the i7, I don't think you'll change much by swapping to the i5. Also, the battery gave out shortly after 2 years.
As a final note, I tinkered with about 20 laptops total and only one laptop had good cooling and high performance, an 17" Asus ROG G55VX with dual fans. But that thing was huge and its successor got trimmed to a single cooler and weaker GPU for a smaller overall size. Good internal design is rare in a laptop.
Edit: even formatting is difficult at dell.