Short version: Get the fastest quad-core for your chipset and with the latest BIOS, and look up reviews to overclock it as far as practically possible with whatever cooling you can afford. Yeah, it stuck that they aren't being produced anymore, you have to settle for refurbished or used.
Personal suggestion: Keep it simple by saving up your money and make a more legit upgrade in four months for the new Haswell generation Core i 4000 series, even the cheapest models will obliterate what you currently own.
Longer version: A lot of similar posts lately. I made
this post for someone else recently asking about using their Core 2 Quad generation processor with any modern 600-series high-end,
it is possible but the CPU must be overclocked. For reference, a older GTX280 will perform like a modern GTX650, to give you an idea of what to shoot for. Don't just look at the number of CUDA cores or frequency or memory interface independently, technology has not scaled equally in the last 5-6 years. That is why you must consider hardware reviews.
Just so you know, your central processing unit (CPU) runs everything in the system, and tells all other components what to do, including telling the graphics card what to draw. Games doesn't run off the graphics card, therefore you can't just upgrade one part and hope for the best, there are limits and depends on number of cores, stock or overclocked. Your stuff is old enough that you can upgrade graphics card(s) too far, where only a brand new CPU/mobo/RAM type will help.
But considering performance equivalencies, don't spend too much just to keep what you have where something newer would suffice. The Core 2 generation is behind approximately 50-60% per GHz of the latest Core i5 3000 series quad-cores in LGA1155 sockets, and since the cheapest is available for $180, don't spend more than $120 on an LGA775 quad-core. Unfortunately, you'd need around $100 for a new board and $25 for new DDR3 1.5v dual-channel kit. Worth is up to you.