It all depends actually. Yes, in some situation you can have memory overcommitment ratio up to 1:2, but you definitely require a lot of memory for this and a lot of similar VMs with low memory usage.
Since ESX(i) 4.0 requires 2GB of RAM for itself, you have 6GB available for VMs when 8GB is installed on server.
In my case I have 32GB of RAM and overcommitment about 14GB, so 32+14 / 32 = 1.44. But my VMs are servers with various OSes, Windows 2k3 32 and 64, some XPs, some Vistas, RHEL 4 64, RHEL 5 32 and 64. Such variety reduces \maximum overcommitment ratio.
VMware vExpert '2009