Here are the few things to be questioned..... with consideration to the sizing of ESXI host.
1. How many concurrent connection of desktop's will be used?
2. What is the configuraiton (CPU, Memory) for those desktops?
3. How many machine are to be dedicated connection to users and how many to be pooled desktop?
4. What is the OS to be used for those desktops?
5. Verison of Xendesktop and its compatiblity with VMware
6. Permission for Xendesktop director to vcenter.
You may also want to watch this excellent boot camp here http://communities.vmware.com/community/vmtn/desktop/view/bootcamp