I use an hp x58 i7-920. The only way to overclock is use one of the software overclock tools like SetFSB if you can figure out which PLL is on the board. I don't recommend trying. The Dell shipped BIOS does not have any of the OC options you need. The case will not support a MB upgrade (the atx MB panel is tack welded into the case and the case headers use a custom plug rather than individual wires so they will not fit on a new MB. Ask how i know.)
The bios on the MB may or may not support an upgraded processor. Not all processors will be supported.
The x58 chipset MBs supports processors with performance equivalent to a modern i3..... unless you are transcoding or ripping video where the i7 quad core hyperthreading and triple channel memory make an x58 system a BEAST better than an i5.
IF you get the used server plan on running it at stock clock, plan on adding a strong video card and gaming happily for years. Just make sure the refurb server comes with a 64-bit version of windows. Also make sure the server has a good cooling solution, that gen i7 used a lot of power even at stock clock. Some servers get cooling at the expense of lots of noise. not a good trade.
Update: I just looked at the specs ... 1x Intel Xeon W3505 2.53GHz Dual Core CPU, ... run and hide from this processor. A weak i3 will kill it. dual core and slow.