Try something like this
Get-Datacenter | ForEach-Object{
$cluster = Get-Cluster -Location $_
$esx = Get-VMHost -Location $_
$_ | Select Name,
@{N='Cluster#';E={$cluster.Count}},
@{N='CpuGhz';E={[math]::Round(($esx | Measure-Object -Property CpuTotalMhz -Sum | Select -ExpandProperty Sum)/1000,1)}},
@{N='MemGB';E={[math]::Round(($esx | Measure-Object -Property MemoryTotalGB -Sum | Select -ExpandProperty Sum),1)}}
}