You can run max of 320 VM's in a single server with ESX 4 .
But in your case you need HA, so we need to put the ESX in HA cluster .
You can run max of 160 VM's in each ESX (ESX 4 with Update 1)Server in HA cluster. So my recommendation is to use 3 ESX in a HA Cluster.
Each will run ~60 VM's in case of any ESX fails remaning ESX will take over the VM's from the failed ESX.
So configure No of Failover host to 1 during HA configuration.
Recommended H/W: HP Proliant DL385 G6 -3 No.s
with 2X6 core processor
96 GB of high performance RAM.
2X 72 GB internal SAS HDD 15k RPM.
To use HA you need to have shared storage (NAS or SAN) for VM’s VMFS datastore.scale the datastore size according to your requirement
Use VMware vSphere 4 Essential Plus license
Note:-
If your VM’s are processor intensive then go with HP Proliant DL 585 G6 each with 4X 6 ocre processor. And also change the license to vSphere 4 standard
regards
Manic