For the benefit of anyone reading and wondering, I got my 560Ti OC installed yesterday and it's running fine. I had no problems even while standing outside the Stormwind AH in World of Warcraft with all settings cranked to maximum, and the system temp didn't even rise. (Not that I got 60fps - I averaged 34 - but still.)
I plan to run a couple more tests using Borderlands and Rift to make sure it's happy under load, but if a 520W supply can cope with a 560Ti on my system - which has three HDDs, an SSD, a DVD-RW and two 300mm fans to cope with - then it should suffice for most other systems as well.