i recently upgraded from an x4 620 to a 2500k@4,2 and it's running pretty nicely, coupled w/ a new hd 7790 gcard upgrade, I feel i can turn up the graphics setting on games a notch or two. other than that, daily stuff feels faster as well, but to be honest, my old x4 620 runs everything pretty decently, but I just got the upgrade itch and bought a 2500k+board off ebay for 200. the new 4xxx+board is like 335 on sale at the time, I saw the benchmarks, not sure it's like 50%+ faster than 2500k to warrant the price difference.
plus i got lucky, this 2500k can do 3,5-6 on stock volts, but I just felt 4,2 is plenty for what i do, did about 12hrs linpack on 3,5 tho. not a hickup on stock volt.
so far, I like it a lot, i think 2500k is gonna be like i7 920, gonna last a long time to come. maybe only reason to upgrade it for the future is to get better power management features considering Intel is investing heavily on that now.