While a W3690 Hex will work in a T3500 I chose a X5670 Hex, feeling it was a good compromise in power consumption and other factors.  The X5670, 5680, 5690 all support dual processing, the W3690 does not.   In addition, the X56xx series have a higher max operating temperature and in the little T3500 box I felt that cooling and operating temps needed to be watched.  It's not always the fastest MHz that is the most important, there are other things to be aware of when building a gaming machine.

The X5670 6-core, Geforce GTX 1060/6GB, 24MB RAM in my T3500 easily ran modern games on Steam, like Fallout 4, COD, etc.

See this for comparisons: http://www.cpu-world.com/Compare/858/Intel_Xeon_W3690_vs_Intel_Xeon_X5690.htm,

I chose the X5670 because it used less power (95W) and would not heat up the 525W PSU along with all my other upgrades.  The 6-core X5670 @ 2.93GHz (Turbo@3.3GHz) with 12 Threads is enough to play most games, the lower heat, power needs and finally, the cheaper price put the X5670 in a sweet spot for gaming with the T3500.  Okay, you don't get bragging rights with the max speeds but so what, it works very well in this situation. It's your choice, none of these used CPU's are that expensive.

Don't forget to install a SSD hard drive in there, that will really help with data transfer and create less heat.

CPU GPU prices have come down so you may want to look into those after you've got everything else running.

http://www.cpu-world.com/Compare/859/Intel_Xeon_X5670_vs_Intel_Xeon_X5690.html