I did some looking and so far, the fastest I've seen is the i5-3470S, which is about 50% faster than what you have.
Of course the big difference is going to be the 4 physical cores, which will increase overall speed and multitasking a lot more than just the pure performance jump. The only issue is if there are two different cooling solutions for dual-core vs. quad-core - the newer the systems, the more likely they are equivalent, but 3-series is right on that edge.
You might want to check that out, comparing heatsink-fans from the two different configs (use teardowns, reviews, images, Dell parts, etc.) just to make sure they are the same.
My best advice is to NOT try it.
Keep your working computer the way it is. If you want some extra speed, try installing a SSD (for bootable Windows and all Apps).
I can definitely agree with that, as CPU upgrades easily offer the lowest return on investment of any upgrade - I took a Latitude from a dual-core i5 to a quad-core i7, with benchmark performance more than 2X as high, and I barely notice it.
Those AIOs are also a lot of steps and effort to upgrade and it's very easy to break a clip or forget a connection, and other than adding in memory, you really have to take a large portion of the system apart.
SSD -> Memory -> CPU would be the order of upgrades I would undertake with an AIO.
Before all "merci beaucoup" (thank you very much in French) !!!!!
So, I understand that the best to do (to avoid a waste of time), is to put a SSD instead the HDD, and probably 4GB RAM more (total of 8GB of RAM : I have actually "only" 4GB).
My aim is to format and install a clean Windows 10 instead of Windows 7 64bits.
Even for this (Windows 10), the 4 cores CPU (instead the twin cores CPU) is not so efficient ????
Once again, thanks in advance.