The I7-640m also has two core four thread and only gives you an extra meg of cache and 133 mhz base 266 mhz in turbo over the I5 I recommended for $15, but it loses 33 MHz on the integrated graphics not that it is much of a powerhouse, even a geforce 9400m performs better but laptops don't have much in graphics upgrades.
The p6200 has 667 MHz graphics, 640m 733 MHz, and the 560m 766 MHz. All 3 use a 500 MHz stock clock, and overclock when thermals permit. Either I5 or I7 is huge over the P6200 and I3, but the value is definitely with the I5, allows more money for that SSD
One other upgrade, see if the 4GB ram is a single stick and 800 mhz pc3-6400 or 1066 mhz pc3-8500, max the laptop can hold is 8GB (2x 4GB 1066)