Troubleshooting time from a certified tech 😉
Remove everything non essential, unplug the IDE and Floppy cables, unplug the power from all of the drives. Remove any PCI cards that aren't needed. If video is onboard, rip out every add-in card. Now...try and boot it with one stick of RAM if you used more than one.
If this doesn't work, try clearing the CMOS and try booting again. If this does work, turn it off and plug one thing back in at a time....either one card or one power or IDE cable, booting between each time until you find which card or part gets plugged in and causes it to not work. If it all goes back in, it was likely just a card or something that needed to be reseated.
If that doesn't work...I'd say try a new Power Supply, but most Dell's are wired differently and need a Dell PSU so unless you can borrow one from a friend you're out of luck here.
If you can get another PSU and it doesn't work, it's probably the CPU or the motherboard. At this point it's hard to say what's wrong and swapping a known good CPU into a possibly bad Mobo isn't the best idea...though sometimes it has to be done. Same thing with possilble bad CPU into known good Mobo.