Yes the Intel X5670 comes with HT support.
We are a Dell shop and have plenty of R710 in the past. In the Dell BIOS they dont use the term "Hyperthreading" and name it "logical processor" instead. So please check this.
On paper the R710 with x56xx (Westmare) supporting up to ESXi 6.0 but also 6.5/6.7 will run. You cant install/run 6.7 on a x55xx (Nehalem) based CPU.
Dell removed all Update ISO and also the direct download through LCC isnt working anymore. Consider to check for BIOS updates manually by visiting dell.com/support. They released Spectre/Meltdown related BIOS upgrades.
Regards,
Joerg