I've had this happen on a PE650 recently, it turned out it was the CPU that developed a fault. They replaceed the motherboard, PSU, RAM and riser card before tracking it down! The symptom I had was nothing ever displayed to the monitor but it did detect an input source for a couple of seconds at the start of a boot attempt (LED on monitor goes from amber to green then back to amber).
I think the heartbeat display is just power indication isn't it?
Dell will insist you try and reseat everything first (and it makes sense to, this includes hard drive and CD cables). The VGA card is a bit odd as it come off the PCI riser card (at least it did on my system). The odd thing was this server was working fine until it was rebooted.