Processor upgrade on a dell precision 390?

Possibly. But unless you find your 6750 used at a bargain price, it would be a waste of money. You won't notice that much difference in performance. I would put the money in a better video card instead, such as the 8500gt, which the dell power supply should be able to handle. Or, possibly a memory upgrade. 4 gb kits are about $38-40 at frys. When I upgraded my compaq pc, I changed out the motherboard and reloaded windows. But dell cases sometimes make this more difficult. It all depends on your backplate design.