Hi, I have just reinstalled my PC and now catalyst can't find my native resolution whereas it could before. The native resolution is 2560 x 1600 60hz DELL U3011 and it won't go above 1920 x 1080 60hz

It reports my monitor as 1600 x 1200 75hz which is very wrong

My specs are:
Mainboard : ASRock Z68 Extreme4
Chipset : Intel Z68
Processor : Intel Core i5 2500K @ 3300 MHz
Physical Memory : 16384 KB (4 x 4096 DDR3-SDRAM )
2*Video Card : AMD Radeon HD 6900 Series (Crossfire)
Hard Disk : OCZ-VERTEX3 ATA Device (240GB)
CD-Rom Drive : TSSTcorp DVDWBD SH-B123L ATA Device
PSU: HX850W Corsair
Win7 64 bit

I installed 12-8_vista_win7_win8_64_dd_ccc on pretty much a fresh install of win7 64 bit

any ideas what the problem could be?