It all comes down to how much money you want to spend really. Take a look here to compare CPU's . If you choose to stick with Intel, you might want to look at the i3 and i5's as well. Don't spend money on things you don't need.
You also have to look at the socket type There are two types. LGA 1366 (i7 core) and the 100007671 50001157 600005862 (LGA 1156100007671 50001157 600005864 (i7, i5 and i3 core). Your power supply will need to have a 24pin connector and the CPU connection can be 4, 6 or 8 depending on the board manufacturer. The primary things you need to get for the upgrade is motherboard, processor and memory, other than that you can use everything else from your old system. The real question is how much money are you willing to spend. Give me a ballpark and I can find something for you.
And tell me what you are going to do with it. Overclocking, gaming, bragging rights, etc. etc.