A used 920 is not worth 250 alone, you can bet its been clocked to 4gig (which is not a bad thing), the ram is about 120 new, so about 60nd hand. and the cpu worth prolly 180ish.
Its a fair trade for 250, tho please note, that 6gig of that ram will be basically useless as notyhing needs more than about 4gig.
If you look at the benchmarks for the 920 vs 2600k thier is about a 5% performance difference. in reall world gaming you might just might notice 1/2 fps difference.
I would point out that you need to buy a new mb as well tho, and a decent x58 is still far more expensive than a h68.
Would i purchase it, prolly yes.