what is your budget? i5 2400 is good too.
almost any sandy bridge (save 2390t may be ) or ivy bridge i5 is good for gaming. the advantage of 2500k is that it has unlocked multiplier which allows it to be overclocked.
2500K is still very excellent for gaming.
If you don't overclock than even, as mentioned, the alternatives (i3's, i5-2310 [yes its real, clocked a tad higher than the i5-2300 and at the same price] and the i5-2400) are all good. At the moment, Intel is on top of the upgrade path with their new motherboards, the Z77's. Just look into it.
If you're not going to overclock, an Ivy Bridge processor is slightly preferable to a Sandy Bridge one. IB has trouble managing the heat from extreme OCs, which can allow a highly-overclocked SB to catch up even though it has slightly worse instructions per clock (IPC). But if you're not OCing, IB > SB at the same stock clocks (and it uses a little less power, too, which is nice).
So I'd probably consider a 3550 if I were you. But context matters here--is this for gaming or something else? what's the total budget you have? etc.
No harm getting the i5 2500K even if you are not interested in overclocking presently. It is one of the best all round desktop CPU's till date. Furthermore, its within your budget. And the overclock friendliness is sure to come in handy at some point in your system's lifetime.