I found four 390's that have an I7-2600 on newegg (screengrab below fit 3).  So yes, your 390 can take an I7-2600. But compared to your I5-2400, would only yield 28% better performance according to game-debate.  Would also only be going from 3.1GHz to 3.4GHz.  Unless your a gamer, you'd be hard pressed to notice any difference.  If performance is 2x better, then you'll notice a difference.  Can often be not much going from quad core to quad core. You'll notice more difference with 1. Use an SSD.  They really do breathe new life into older machines.  2. Max the RAM if not already.  It won't cost you anywhere near $200, let alone $100. I use A-Tech RAM, which saves me $ but also has a lifetime warranty.  Crucial also has a lifetime warranty, but their RAM is a little pricey. [IMAGE] [IMAGE]  
I have a dell optiplex 390 I want to upgrade to an i7 2600 from a i5 2400 but last time i tried to upgrade my cpu (i5 3470s) my motherboard stopped working altogether so i wondering if i can get an i7 2600 to work with the dell optiplex 390 motherboard because I dont want to through the hassel of spending like 200 bucks for a a new cpu/mbo/ and ram i just want to buy a used 2600 and upgrade performance  Also i forgot to mention that i have update the Bios to A14 i think thats the latest one
CPU UPGRADE