I would honestly save your money a while longer, till you have about double that much.
There are a lot of things that will prevent you from realizing the value of potential new upgrades. You may have to change out multiple things to see much of any increase.
The slot you are on is on the way out and if you get a new motherboard to OC it would have to still be one that is obsolete by today's standards with no future potential to move to Ivy Bridge.
I would say if you are getting a new motherboard to make it a Z68 with a 2500k so you can OC to your heart's content, but then you would have to pay $100 for a Windows 7 Upgrade CD so you could have an OS on the new setup.
All that together is like $400, maybe less by the time that you save that much. If you don't decide that you would rather have Z77 and Ivy Bridge instead which would keep the price about the same.
I don't know if the Inspiron 580 uses a universal case, so it could require a new case for $60 too. The CPU Cooler might also not fit a 2500k, I am not familiar with that specific model.
I don't know how much performance increase you would really see from doing all that, because the video card is where most people "feel" their upgrades.
Unless you are doing a lot of processor intensive tasks like graphics/sound editing and relatively little gaming, then the 2500k setup would be a whole lot better.
The point is, I would suggest you save however long it takes to accumulate $350 - $450 in total and then come back in here and ask the same question.
That way we aren't quite so limited in how we can help you and how much gain you can get for each dollar spent.