I would really like to thank the OP for posting his build. It informed me of the possibility of adding a new fan (I ended up cutting the back fan wires and splitting them so I could power both) by removing the HDD frame, but the PWM information was precious!
This computer was going to the trash bin (really) according to my (not-very-smart) former boss, so I took it, dumped the HDD, and put in a new Crucial 240GB SSD. At the time I was short on cash so simply updated the graphics with a Zotac GT 1030 - two years of happy potato gaming ^^
From the original Dell build I updated the BIOS, recently added more RAM for a total of 16GB, installed an i7-2600, and replaced the PSU with one that could provide more than 146W on the 3.3 and 5V rails (following the advice from user Speedstep on the official Dell forums).
Now I'm just waiting for a GTX 1080 to arrive (let's see if I can still access the SATA connectors!)
Thanks, asand1 !