I have an Asus P6X58D Premium Mobo, with at SATA III controller in it as well, its a Marvell controller, not sure what model though, I recently bought a Corsair Performance 3 SSD SATA III, hooked up to the SATA III controller on my mobo WITH a SATA III cable, and I am still only getting SATA II seq r/w speeds. I've been doing some research and can't seem to find another cause for this but the Marvell controller. So I don't think you should waste your money on a SATA III SSD, unless you are going to upgrade your system to a SandyBrigde build or something, I am probably going to return my SSD, too bad, I really like build, disappointed to find out that SATA III controller on this mobo is crap!
Win7 64bit Ultimate
Intel Core i7 930 @ 4.2Ghz
Corsair Dominator RAM DDR3 @ 1600 Mhz
Asus P6X58D Premium Motherboard
Seagate 500GB HDD
Corsair Performance 3 SATA III SSD
ATI HD 4890 Graphics card
Antec 850W PSU