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A CPU upgrade will help if what you're doing is CPU-bound. For a lot of what one might call "ordinary office-y stuff", a better upgrade might be to change out the hard drive for an SSD. (and, making sure that there's enough memory installed.) So, what is your friend using the machine for, and what is his hardware bottleneck?
 
Oh he's definitely gone already through every other upgrade (ssd, ram, etc.) and he's getting info on the CPU to improve the performance of that machine at the best. No specific bottlenecks but more of a total-upgrade exercise 🙂
This said, i7-870S seem very rare to find, but there's some i7-860S on eBay and they should fit in the "low power" scenario that, on that machine, prevents using non-S i7 CPU's.
 
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