OP: For $400 you have these choices.
1. Two AMD Hawk 6870's
2. Two Nvidia TwinFrozr II 560's
3. One GTX 570 Lightning
. Lightning has no issues. Don't let anyone tell you other wise, there lying or don't know any better.
4. One AMD 6970 Lightning
5. One 'OPEN BOX' GTX 580 if your patient and can order from Newegg in Australlia.
These options are your strongest options for $400 usd. I see your on one monitor with 1000 watt PSU.
If you SLI/Crossfire right now your closing the door for future upgrade but you will have a good set up.
If you buy one of the higher performing options you can SLI/Crossfire later down the road and increase your performance even more so then the SLI/Crossfire options now. Prices will also drop in a couple of years which will be even a better price/performance gain and you'll get a new bump which will keep you going a couple years more.
Two 6950's will be over your budget price and to have one 6950 now isn't better than those above options in performance.
Two GTX 460's are beat by two GTX 560's in performance even if it is for less money still under your budget.
One GTX 480 is equal to one GTX 570 or AMD 6970 and I'd rather go with more update video card if I was you with current series.
Don't buy a 6950 with the hopes of unlocking shaders or flashing bios to 6970, because vendors are changing this and there is no guarantee you'd end up with one. Not to mention the risk involved which I'm not sure you'd want to take. Great over clock cards as is but the unlock is not the reason to buy one any longer. More importantly not higher in performance than those above options without successful unlock.
Personally I'd go with a single Nvidia GTX 570 Lightning card option, the upside, small one, is you can use that GTX 260 for a physx card. That however will end when you eventually SLI another 570 card though. My choice for you
Now this is my opinion which from these replies you can see vary. Which you'd want to choose is your option. Both companies have their niche'. Your one monitor 1920x1080p is not going to require more VRM and won't increase your performance.
Ball is in your court as you do your own homework and choose what's best for you. I've narrowed it down for you and explained why.