Project Euler and PowerShell - Problem 4

Problem 4: A palindromic number reads the same both ways. The largest palindrome made from the product of two 2-digit numbers is 9009 = 91X99. Find the largest palindrome made from the product of two 3-digit numbers.

function Reverse-Integer
{
 param([string]$str)
 for ($i = $str.ToString().Length - 1; $i -ge 0; $i--)
 {
     $c = $c + ($str.ToString().Substring($i,1))
 }
 [int]$c
}

$big = 0
for($x=100;$x -le 999;$x++)
{
 for($y=100;$y -le 999;$y++)
 {
     $xy = $x*$y
     $yx = Reverse-Integer $xy
     if ($yx -eq $xy)
     {
         if ($xy -gt $big) {$big=$xy}  
     }  
 }
}
$big

This one takes a long time to run. If anyone has any ideas on how to refactor this, please share!