A Precision 390 appears to use a motherboard that is a Dell specific variant of the BTX form factor, & any auxiliary circuit boards connected to it (such as the front panel connector/power switch board) are also Dell specific - indeed probably Precision 390 specific & connected by Precision 390 specific cables. The PSU may also be Dell Precision 390 specific, although I suspect it is probably not. .So the only sensible replacement motherboard board is a Dell Precision 390 motherboard, with compatible old Core 2 CPU that will probably just manage run Windows 10 very badly.
As an upgrade project a BTX type chassis is really a non starter as this is not a common form factor, & repairing it is not financially viable unless you already have the parts & are fitting them yourself. I'm afraid I think you need a different project - preferably one with an ATX format case.
(can you tell from the above that in the past I have actually repaired/upgraded a couple of old Dell OptiPlex machines using slightly later Dell motherboards only to find on further checking that other boards & cables in the cases were not compatible with the "new" motherboards & had to be replaced with versions compatible with the replacement motherboard)
Edited by rqt, 17 May 2016 - 03:06 AM.