Noticed that performance of virtual terminal servers was poor running on HP DL580 G5 servers (4 x 3GHz Intel CPUs, bought May 2008) with ESX 3.5/vSphere 4.0.
Noticed that Performance was excellent when running on Dell R710 with 2 x Intel X5560 (Nehalem) CPUs. Turns out that this is less to do with XenServer and more to do with the EPT-D function present in the X5560 CPUs. The XenApp optimisation setting for VMs within XenServer is effectively a software implementation of this EPT-D function. AMD have an equivalent feature called RVI.
See Virtual Reality Check Phase II for more info.
Thus I would say that for acceptable virtual Terminal Server performance (i.e. not noticably different from running on physical servers) you really need this CPU feature present.