I differ in my opinion from akxpckwb in what you should get. I think you should keep your graphic card and upgrade it only after upgrading the cpu, motherboard (MB), and RAM, only if the cpu/mb/ram upgrade by themselves aren't enough to satisfy you.
For the CPU I recommend the Intel Core i5-2500 Sandy Bridge 3.3GHz (3.7GHz Turbo Boost) LGA 1155 95W Quad-Core Desktop Processor Intel HD Graphics 2000 BX80623I52500http://www.newegg.com/Product/Product.aspx?Item=N82E16819115073
For the MB, a good mid-range choice is GIGABYTE GA-Z77X-D3H LGA 1155 Intel Z77 HDMI SATA 6Gb/s USB 3.0 ATX Intel Motherboardhttp://www.newegg.com/Product/Product.aspx?Item=N82E16813128546
and for memory, a good inexpensive 8 GB set of ram would be G.SKILL Ripjaws Series 8GB (2 x 4GB) 240-Pin DDR3 SDRAM DDR3 1600 (PC3 12800) Desktop Memory Model F3-12800CL9D-8GBRLhttp://www.newegg.com/Product/Product.aspx?Item=N82E16820231314
Put it all together, and if you still don't get the performance you want, add an AMD based video card, model 6870 or higher to your system.