I was working on a PVS deployment recently and needed to quickly add some RAM to ~200 PVS streamed VMs. To automate this task, I put together the following PoSH script that combines PowerCLI and the XenDesktop PoSH SDK to add RAM to all machines in a particular desktop group:

###################################################################
#
# Change-VM_Memory_CPU_Count.ps1
#
# -MemoryMB the amount of Memory you want 
#  to add or remove from the VM in MB
# -MemoryOption Add/Remove
# -CPUCount the amount of vCPU's you want 
#  to add or remove from the VM
# -CPUOption Add/Remove
# -DesktopGroup the XenDesktop Desktop Group to run against
# -AdminAddress host name of the XenDesktop DDC to run against
#
# Example:
# .\Change-VM_Memory_CPU_Count.ps1 -vCenter vmvcatl05 -MemoryMB 1024 -MemoryOption Add -DesktopGroup 'All User Windows 7' -AdminAddress CTXXDATL01
#

#
####################################################################

param(
    [parameter(Mandatory = $true)]
    [string[]]$vCenter,
    [int]$MemoryMB,
    [string]$MemoryOption,
    [int]$CPUCount,
    [string]$CPUOption,
	[string]$DesktopGroup,
	[string]$AdminAddress
)    

function PowerOff-VM{
    param([string] $vm)

    Shutdown-VMGuest -VM (Get-VM $vm) -Confirm:$false | Out-Null
    Write-Host "Shutdown $vm"
    do {
        $status = (get-VM $vm).PowerState
    }until($status -eq "PoweredOff")
    return "OK"
}

function PowerOn-VM{
    param( [string] $vm)

    if($vm -eq ""){    Write-Host "Please enter a valild VM name"}

    if((Get-VM $vm).powerstate -eq "PoweredOn"){
        Write-Host "$vm is already powered on"}

    else{
        Start-VM -VM (Get-VM $vm) -Confirm:$false | Out-Null
        Write-Host "Starting $vm"
        do {
            $status = (Get-vm $vm | Get-View).Guest.ToolsRunningStatus
        }until($status -eq "guestToolsRunning")
        return "OK"
    }
}

function Change-VMMemory{
    param([string]$vmName, [int]$MemoryMB, [string]$Option)
    if($vmName -eq ""){
        Write-Host "Please enter a VM Name"
        return
    }
    if($MemoryMB -eq ""){
        Write-Host "Please enter an amount of Memory in MB"
        return
    }
    if($Option -eq ""){
        Write-Host "Please enter an option to add or remove memory"
        return
    }	
    $vm = Get-VM $vmName    
    $CurMemoryMB = ($vm).MemoryMB

    if($vm.Powerstate -eq "PoweredOn"){
        Write-Host "The VM must be Powered Off to continue"
        return
    }

    if($Option -eq "Add"){
        $NewMemoryMB = $CurMemoryMB + $MemoryMB
    }
    elseif($Option -eq "Remove"){
        if($MemoryMB -ge $CurMemoryMB){
            Write-Host "The amount of memory entered is greater or equal than 
            the current amount of memory allocated to this VM"
            return
        }
        $NewMemoryMB = $CurMemoryMB - $MemoryMB
    }

    $vm | Set-VM -MemoryMB $NewMemoryMB -Confirm:$false
    Write-Host "The new configured amount of memory is"(Get-VM $VM).MemoryMB
}

function Change-VMCPUCount{
    param([string]$vmName, [int]$NumCPU, [string]$Option)
    if($vmName -eq ""){
        Write-Host "Please enter a VM Name"
        return
    }
    if($NumCPU -eq ""){
        Write-Host "Please enter the number of vCPU's you want to add"
        return
    }
    if($Option -eq ""){
        Write-Host "Please enter an option to add or remove vCPU"
        return
    }

    $vm = Get-VM $vmName    
    $CurCPUCount = ($vm).NumCPU

    if($vm.Powerstate -eq "PoweredOn"){
        Write-Host "The VM must be Powered Off to continue"
        return
    }

    if($Option -eq "Add"){
        $NewvCPUCount = $CurCPUCount + $NumCPU
    }
    elseif($Option -eq "Remove"){
        if($NumCPU -ge $CurCPUCount){
            Write-Host "The number of vCPU's entered is higher or equal 
            than the current number of vCPU's allocated to this VM"
            return
        }
        $NewvCPUCount = $CurCPUCount - $NumCPU
    }

    $vm | Set-VM -NumCPU $NewvCPUCount -Confirm:$false
    Write-Host "The new configured number of vCPU's is"(Get-VM $VM).NumCPU
}

#######################################################################################
# Main script
#######################################################################################

$VIServer = Connect-VIServer $vCenter
If ($VIServer.IsConnected -ne $true){
    Write-Host "error connecting to $vCenter" -ForegroundColor Red
    exit
}

if($MemoryMB -or $CPUCount -ne "0"){
    foreach ($vm in get-brokerdesktop -DesktopGroupName $DesktopGroup -AdminAddress $AdminAddress -PowerState Off)
	{	
		$vmwvm = Get-VM -Name $vm.HostedMachineName
		if ($vmwvm.MemoryMB -lt 4000)
		{
        	if($MemoryMB -ne "0"){
	            if($MemoryOption -eq " ") {Write-Host "Please enter an option to add or remove memory"}
	            else
				{					
	                Change-VMMemory $vm.HostedMachineName $MemoryMB $MemoryOption					
	        	}    
			}
        }

        if($CPUCount -ne "0"){
            if($CPUOption -eq " ") {Write-Host "Please enter an option to add or remove cpu"}
            else{
                Change-VMCPUCount $vmName $CPUCount $CPUOption
            }
        }

    }
}

Disconnect-VIServer -Confirm:$false