Prime 95 is the only way to go. they have a multiple core x64 version so make sure you get the right one. also to monitor all of my core temps and not have some big fuckin application running i use coretemp. it's tiny and it shows the temp of your cores and can monitor fan rpm and whatnot. i just keep it in the system tray.
I would run a prime95 stress test overnight and see if your machine is locked up in the morning. if it isnt, you have a solid machine. if it does, you may have some tweaking to do.
memtest86 is also good for testing your memory to make sure you dont have a bad stick. better to check that shit while you can still RMA.