First off, if you upgrade the processor, you are going to need a new motherboard.
A GTX 460 will be good enough when overclocked. http://www.bit-tech.net/hardware/graphics/2010/07/21/overclocking-nvidia-s-geforce-gtx-460/1
Here's a good one. http://www.newegg.com/Product/Product.aspx?Item=N82E16814162055
Only $100 after rebates!!!!
If you want, add an ssd with the money you save. That will give you a huge system boot time and application launch time reduction. http://www.newegg.com/Product/Product.aspx?Item=N82E16820227725
That's is a SATA III drive if you have sata 6.0gbps on your motherboard. It is backwards compatible with SATA II/SATA 3.0 gbps
So, together those are about the price of an HD 6950.
If you want a cpu and mobo upgrade, i have links.
motherboard: ASUS P8Z68 LEhttp://www.newegg.com/Product/Product.aspx?Item=N82E16813131773
you can't crossfire nor sli, but your psu does not have enough power connectors for that anyway.