|"so i gotta get a new atx psu?"|
No, you've got to explain what you're doing & how, plus improve your troubleshooting skills. Are you 100% sure the MSI setup works? Board, CPU, RAM, video are all "known to be good"? Are you 100% sure the Dell PSU is a standard unit & not a proprietary unit? If it's a "Dell only" unit & you connected it to the MSI board, you more than likely fried the board.
Have a look at the main ATX plug. I don't know if it's a 20-pin or 24-pin, but make sure the wriring matches the following, if it doesn't, it's a "Dell only" unit & your MSI board is probably dead.
Standard 20-pin ATX pin-out
Dell 20-pin ATX pin-out
24-pin ATX12V pin-out