http://tools.pingdom.com/fpt/
you could use that to test responce times. but then again it aint a script you can use on ur own site
as Sillysoft said you could use snmp or dll/api using com objects in PHP to get memory/cpu usage etc.
If you want to see how fast a script runs you could create a dummy script that has a few loops and before and after the loop use the php function microtime() and then do a comparsen and then you can see the speed in milli seconds a script runs on your server