I've read that a 5970 ist just about as much as a 1090T can handle and a cross- resp. trifire with another card would cause the 1090T to seriously bottleneck and therefore would not be a worthwile consideration. Now I've seen on youtube someone building a 1090T rig with two 5970s in cross- resp. quadfire but he didn't show any performance tests like benchmarks and also didn't comment on it.
So, would an overclocked 1090T, maybe to 4.1Ghz, still be too weak for a 5970/5870 crossfire combination?
1090T @4.1GHz Motherboard
ASUS M4A89TD Pro/USB3 Memory
4 x 2 GB DDR3 1600Mhz G.Skill Graphics Card
XFX 5970 Black Edition Hard Drive
1x X25-M 80GB 2X SpinPoint F3 1TB Power Supply
Corsair HX 850W Case
HAF 932 CPU cooling
Noctua NH-D14 GPU cooling
Windows 7 64 Bit Professional & Win XP 32 Bit Pro Monitor
Hanns-G HZ281HPB 1920x1200 27.5"