It looks like from what I can see the i7-2600 should work and is what it may have come with. Its a 65w TDP limit so any 2nd gen 65W chip should work.
Hello! That is generally all you can do and that is if the BIOS will accept anything that Dell hasn't tested with it plus you have to stay in the generation of your motherboard and wattage it will accept (65W I believe). BTW the series up until the latest generation is i3, i5, and i7. No i6 🙂
If your running an i3 model, you could try for a quad core i5. Those tend to be a good, cheaper middle ground.
No thats a 1st Gen processor and probably will not work. You should look for an i5 2400. From my google search it looks like that is what your computer came with when an i5 was ordered with it. Putting anything else in may not work.