benberg2013 :
Bought this dell latitude 6420
with i5, 8gb ram, 320gb hdd.
costed 188GBP. Is this a good price?
How much will cost to upgrade to i7? what will i gain by this?
Hard to see much of a benefit. First, you may have the BGA version rather than the PGA version, in which case there's no upgrade that's feasible. If you do have one that can be swapped out, it'll be guesswork if an i7 will even work, given you'll have an old, poorly documented OEM motherboard in a laptop. And the price will be guesswork as well, since you'll essentially be relying on ebay sellers that have salvaged the parts since there was never a retail arm for these processors.
Even getting through all this, the i7s that have the best chance to work in this socket are the ones that are also 35W like your i5-2520m, which means they will also be 2 core/4 thread CPUs, not 4 core/8 thread CPUs like the 45W Sandy Bridge mobile I7s. In which case, it'll be a "who cares" type of improvement.