Something's wrong here.
1) Your Sony's maximum resolution is low for a 5970. If you want to finish your build now, either downgrade the GPU to just one 5870 which will max out every game on your Sony (anything higher won't be detected by the human eye), or stay the 5970 course and get either a 2560x1600 display or 3 displays at 1920x1200 resolution.
If you are willing to postpone your final build, get a 768GB 460 and keep the display for now. It will perform slightly behind a 5870 on 1080P for less than half the price. Then replace it with the ATI 6xxx series when that comes out after the summer along with a newer display as you will have plenty of options then. A good GPU without a good display is like a good engine without wheels. right now, your 5970 is a V12 engine and your Sony is the 6" rim.
2) If you have the money, always go for the cool, elegant, quite and power-efficient high-end single card solution. Too many people here recommend SLI or crossfire. If computers had emotions they would be crying reading these comments. If you install one card you will get 100% performance. If you install two you won't get 200% performance.
3) I hate this part and I ask you to go to all hardware websites (tomshardawre, anandtech, bit-thech.net, ect.) and don't take my word for it but, 480 has got nothing on the 5970. It performs less, generates more heat and is louder. I wouldn't recommend it to friend or foe.