Thanks for the pointers Dave.
I've been able to use VT on this setup.
In fact, due to the location of this system, I don't really need it, because the server is standalone (no network storage) and I only have one network cable to plug in it.
But what I've done is, I created a tiny VM to use as router, created an internal only vswitch, for all other VM, and setup RDR/NAT rules on the router standing between the physical network and the ESXi internal network.
This way, the ESXi has no direct access to the physical network. It's effectively locked in side itself.
Hard to explain, but a great fun to do
PS: Didn't tried 64bits VMs though, maybe I'll give it a shot when I get time, quite busy at work right now.