i3-2xxx/3xxx: dual-core with hyperthreading
i5-2xxx/3xxx: quad-core without HT
i7-2xxx/37xx: quad-core with HT
As for whether or not the extra processing power is an advantage or not, that depends on whether or not the software or software mix you use constitutes a large enough workload to justify the extra cost in your budget.
I look at it like buying stuff that is "on sale": an advantage/discount is really an advantage/discount only if you actually end up using/needing what you buy. If you buy into a $100 more expensive CPU but end up not using software that can leverage the extra processing power to an extent that feels like it was worth $100, you wasted $100.
You want a SSD too, good mobo, and fast CPU. CPU alone can only do so much.
Ever since I upgraded to SSD, 3570k, and Z77 board, my gig boots up in 12 seconds. 1 second to log in and another 5 secs to launch COD. It's ridiculous I know.