Hi...
Do u mean that the machine isn't starting up at all..or it's powered on but the boot doesn't finish...??
In the case of not starting up at all..and according to the following link (Note section before additional info):
All memory of the VM must be reserved..i.e. if ur VM will have 2 GB of RAM..they must be reserved on the host.. which means that the host must have 3GB of RAM + (2GB reserved + 1 GB for host itself).
VMware KB: Configuring VMDirectPath I/O pass-through devices on an ESX/ESXi host