Sounds like motherboard is toast if I had to guess offhand, based on previous symptoms. I'd do a full diagnostic before buying a new board if I were working on it for you but you probably lack the equipment to do so yourself. Dell mobos (and OEM PC mobos in general) are difficult and expensive to replace anyway; you could probably gut the thing and get a new case, motherboard+processor combo kit and some new ram if the stuff you have is out of date and be in good shape for a lot cheaper than a new PC (especially since you just got a new PSU). Just make sure whatever you get is compatible with your current stuff. Depends on how old your stuff is and what you're looking for in a PC but if you just need something running on the cheap consider it as an option. 