The T3500 can run all the 3600,5600 series and 3500,5500 series Xeons. If you have the aluminum heatsink you will need the heatpipe cooler. The fan is mouted to the case in front of the heatsink. With the HDD tray and the blue plastic RAM cover in place there is a duct formed around the heatsink. It's simple to add a 92mm fan if you want to. The popular CPUs are the 4 core X5687 3.6GHz, or the X5680/X5690 6 core Xeons.
Here are the builds for that.
https://www.userbenchmark.com/System/Dell-Precision-WorkStation-T3500--/2522
Anything scoring in the 70% and up with a W3680,W3690 CPU is probably an overclock. Otherwise they would score the same as an X5680/X5690.