Did you replace the system board and power supply yourself or did Dell send a tech to your house? If they sent a tech I hope he pulled everything when testing the system board. If not I would pull all expansion cards. This time of year especially in the southeastren part of the US lightning goes through the phone line and takes out your modem. Pull all expansion cards and test. Also make sure nothing is connected to the rear of the system. Then you will need to pull all drives... HD, Cdrom etc. Test, if nogo then pull the memory listen for the no memory beeps the system will give. These are all some basic steps to take. If you get this far and the system still dies with out any beeps I would call back in to tech support.

so if I do all that and still nothing what could it be then???  any ideas?  I am going nuts here!

Also try plugging it in directly to the wall jack instead of the surge protector. If everything is pulled including the memory and the system still dies, then they may have to "party" pack it. Where they replace all the parts at once. Because there is always that chance that when you replaced the motherboard a bad part was put back on the board and took it out again.

If they do replace all those parts again, the system needs to be tested first before connecting all the drives, expansion cards etc again. I have heard of people installing bad modems back on the system board and taking the board out over and over again. If you don't have onsite service it might not be a bad idea to pay for a tech to install the parts they send this time. Just a suggestion.