Tri SLI with 280s!!!?? In some cases of poor driver support you get worse of a performance than dual SLI. Read this article before getting your 3rd 280. I am sure some driver updates have improved the performance a bit but still...3 GPUs are pretty worthless...specially if you are going with a graphics beast such as the 280.
Are you gaming at 2560x1600? Just curious. I think that you can put that money into something else like a Q9650 or a top end audio card or a 790i with DDR3 and get a lot more performance (*Edit: and upgrade potential) for your money. Maybe you don't have a budget but I would exclude your 3rd GPU and get better components that will make your system faster. Just my opinion. Also, don't skimp on the PSU quality. You will have tons of issues down the road with a high wattage system and a low grade PSU.
"Alright, the conclusion really doesn't need to be very long. Money wise high-end SLI and 3-way SLI doesn't make much sense, and it never will. It's bragging rights, and only for very few of you with a 30" monitor actually a wish. See, the GeForce GTX 280 by itself already is a power house.
So as much as I'd like to validate the existence of setups like this, common sense always kicks back in and says: dude .. performance versus money wise you better do something else with your money. And that's the cold harsh truth. This setup as presented today doesn't make much sense when you relate it to the core cash you need to drop."