GTX470 3-way SLI scales nicely as evidenced in this guru3d's review:
The driver used was 197.41. If that old driver could provide such nice scaling, newer drivers will provide even (slightly or much) better scaling.
And as pointed out in this review, it is not the scaling that is of concern. It is rather some CPU bottlenecking, which is less at 2560x1600 and relatively more at 1920x1200. Guru3d used i7 965 at 3.75Ghz which is similar to OP's i7 920 at 3.8GHz. However, when bottlenecking does show up for a few games at 1920x1200, the fps already is 100+ and so there is no visual difference anyway.
And Corsair 1000W should be enough. Make sure you distribute the 3 cards (GPU1, GPU2 and GPU3) according to the rail diagram below.
Lastly, the main concern is probably be the heat management inside the 932. This is too long a topic to discuss in this post. One thought, for now, is to replace the side panel 230mm fan with some 120mm fans blowing a stronger stream of fresh air to the cards.