GTX570 OC questions

so... I recently installed a Zalman VF3000f cooler on my GTX570, and it seems to be working great (controlling fan via an onboard fan header). And while the prupose was to get the system quieter than the stock cooler (which it did very well!), I also wanted to do an OC on it for those times that I do a bit of gaming.

So far my stats are as follows (from MSI Afterburner):
Voltage 1.05V
Core Clock 899MHz
Memory Clock 2198MHz
Temp idle: 27*c
Temp under sustained load: ~63*c (this is with fans at lowest settings)

My understanding is that this is not bad (and I am pretty happy with it... especially for a Sparkle branded board that was on sale/rebate/clearance... good steal!), but I just wanted to make sure that nothing was wrong with these settings as this is my first attempt at overclocking a GPU. For day to day use I will be running stock.

I am a little confused about a few things:
1) When I was running a higher voltage I would get a GPU throttling message, even when temps were in the mid 50's. Dropping the voltage seems to allow it to hit a higher temp, without throttling. This seems counter-intuitive to me, and I was under the impression that these cards could hit 90*c or higher without immediate damage, so why the early throttling?
2) When benching for stability is there a particular way to know if it is the core clock or memory clock that is failing other than trial and error?
3) How does this stack in the grand scheme of things? It seems a huge improvement over the stock settings (tests were getting 16fps stock, and now 21fps with the OC, which is a 1/3rd improvement)
4) Any other advice would be welcome as this is a new experience for me.