Considering the relatively small performance boost you'd see from upgrading your CPU, I would probably advise against bothering, especially as the prices those CPUs often command on the used market is a lot higher than I would say they are worth. As that machine's a good few years old now and that CPU socket has been defunct for quite some time, you'd also be investing money into a dead-end machine at the end of the day.
If you're looking to boost performance, I would instead recommend getting an SSD - that would make much more of a noticeable difference than upgrading your CPU, and you'll also be able to move it to any semi-modern system as and when you get rid of that Dell so you wouldn't be wasting money in that sense.
