The GTX570 will do more than twice the work of a GTX460.
Personally I don't think the GTX580 is worth the extra beans, but it is slightly better than just an extra 32shaders; it's about 5% faster/more efficient due to the presence of a full architecture (the GTX570 lost more than 32shaders).

While I would suggest some people wait for Kepler, it's really down to what you are prepared to fork out, when you want to, and how much you like your present GPU. If I had a GTX285, I would probably wait, but if I had a GTX460 I would be inclined to move on. I'm keeping my GTX470's for now at least.

We can only speculate when Kepler will arrive. I think some guesses of an autumn arrival are far too early, and I even think a Christmas arrival is doubtful. I would guess some time in the first quarter of next year, but would not be surprised if we did not see any until this time next year.

Even when Kepler does turn up there will be a period when the developers get the software ready and then another period when they refine the code. During this time (perhaps a month or more) a GTX570 will still be an excellent card and any Kepler's might not be doing much. So add another month or so to the time before we see a usable Kepler. Also worth noting is that the first incarnation of the Kepler will be more like a GTX400 than a GTX500 card. In many respects the GTX570 is a Fermi equivalent of the GTX275. It may be a long time before we see a Kepler version of such ilk. The first Kepler might also be £500+ (560 Euro, $800) and out of the range of most people. So a GTX570 at less than half that price now sounds like a reasonable purchase to me.

Good point. W7 and Vista are the worst for GPUGrid crunching. XP and 2003 servers are at least 11% faster (after full W7 optimization). Linux is usually a bit faster again, but you do need to know what you are doing and it can be a struggle for the amateur Linux user. Cooling is most problematic; some GPU's end up running at over 90deg C (not good for longevity).

That's a sizable OC, so hardly reference. That said you are on W7.
Going by these two tasks, my GTX470 does as well on XP as a GTX570 does on W7:

4091966 2537474 18 Jun 2011 3:55:28 UTC 18 Jun 2011 15:56:51 UTC Completed and validated 38,899.23 38,820.52 44,998.74 67,498.11 Long runs (8-12 hours on fastest card) v6.14 (cuda31)
4094408 2537733 19 Jun 2011 2:22:49 UTC 19 Jun 2011 13:33:21 UTC Completed and validated 38,467.02 38,090.86 44,998.74 67,498.11 Long runs (8-12 hours on fastest card) v6.14 (cuda31)

My GTX470 on an SB system has a RAC of 131,101.68, but it's off for 3h a day, so it could be 150K going 24/7, and that's with the recent low credit tasks. It's at stock and might stick 630MHz but that's about it. All ref cards on the same system and we would have more definitive results, but then some cards clock better than others and that's a factor too; not all GTX460's are the same.

If you look at this task, completed on a GTX570, then you could get over 200K on per day, and that's on W7. Of course what tasks you run makes a big difference, so you would have to go by the same type. Going by a similar task on your system you could get 105K per day. If it was native it would probably be slightly less than half what the GTX570 would get.

Just for Ref. and going by this similar task, a GTX580 on XP x86 could in theory average 295K.