Dell is too proprietary for MB swaps. Not impossible, but definitely rarer.
If you want an I5 4th gen, Optiplex's 3020, 7020, 9020 can all take or have those. Many good deals on refurbs. By the time you upgraded the 780 the way you want, the money is best put toward something a little newer.
- Please kudos posts you like! - Please "Accept as Solution" posts that are a solution. - If you have an Optiplex, stating its model number AND size speeds up troubleshooting. Components vary.