The dark knight is compatible with socket 1155, so no need to replace it.
I would start by replacing just the CPU, board and ram and see how your performance boost goes. If you need more then continue down the line.
I would save the cash on the second 460ti. Unless this card cannot give you the performance you are looking for, and you know the games you want to play use SLI well, you would be better off selling this card (while you can) and purchasing a single more powerful GPU.
Either way of adding an additional 460, or purchasing a stronger single GPU your 550W power supply should be ample if you don't overclock your CPU too hard. Here is a chart with power use for even stronger cards setup in SLI. Two GTX 580's are the only configuration that requires more than 550W output. 460's are hybrids compared to this monster. http://www.tomshardware.com/reviews/geforce-gtx-590-dua...
Also you have your purchase date 60 days out, prices will change ALOT in 60 days as newer more powerful parts will become available, older parts depreciate. This would be a good baseline, but I wouldn't set anything in stone until maybe the week before your purchase.
Thanks for the advice. I do want to change to power supply because its the noisiest part of my PC when not under load. I thought I might save a bit of money by getting a more efficient one too. But maybe I was overdoing it a bit with 850W. Perhaps I'll just start with the board CPU and memory, as you said, and see how it goes.
If your current power supply is too loud, then that's enough reason to opt for a replacement. For your system you should be fine with a 650 or 750. I personally like the corsair HX line as these have never let me down. The HX 750 is 80+ Silver certified, which is about as efficient as a power supply can get.