You really have a sick machine by all but the snottiest standards. Judging by your post count and the fact that most of your time is spent in ATOT and P&N I can understand if you have a pretty snotty standard you like to keep, but if the 580 tells me you are also a gaming enthusiast then someone of your stature knows that a 10% frequency boost is only going to mean a ~5% increase on your minimums in the very best case. Undertaking a platform overhaul for such an incremental upgrade is silly to me, so you should decide:
Are you doing it for performance?
Are you doing it for e-peen or some other nebulous sentiment of power?
Are you doing it for faster peripherals (like you mentioned?)
Honestly I think the performance of a 2500k will disappoint you and probably won't feel any faster than your nehalem. You will have to measure to make sure it's faster, and in a lot of non-gaming scenarios, the 10% clock advantage of SNB will merely bring you to parity with the 4.2 GHz Bloomfield. Your musings on ATOT will post at the same speed, for instance.
If you are doing it for e-peen, or because the chance to run at a nice round number (5.0 GHz) feels good (and it does), then now is probably as good a time as any because you can still fetch ~$250-300 for your current setup which means migrating to SNB will only cost you a couple hundred. Do this before your nehalem loses its value so that something as silly as vanity costs you as little as possible to maintain.
If you are completely unsentimental about your hardware and simply want the best machine for your money and your usage scenarios, stay where you are. Until you say otherwise I imagine your most disk-intensive activity is loading games or apps. So get a Crucial M4, run it on SATA II and get over it. SATA III won't transform your machine's speed nor will a 5 GHz i5 because it's already fast as shit.
I'm not going to go digging around now, but the first round of SATA III expansion cards were built around the Marvell SE9123 family of controllers and were consistently slower than SATA II on the ICH10R. The ASUS Sabertooth X58, for instance, has an ICH10R for SATA II and a Marvell SE9128 for SATA III, and guess what, the ICH10R is faster. So I don't think you'd be satisfied in that regard unless you were getting SATA III and USB 3.0 from an intel ICH. Just wait for Haswell heh.