Mostly agree with above. The fastest mechanical drives can do faster then 150MB/s, so I would use SATAII if possible. Generally 4 drives on SATA2.0 will be faster then two on 3.0. But an SSD will be a much better idea overall. Avg transfers will be slower, but the seek and randoms will be much faster. This is good for OS drives. I'd use a 128GB drive as 64GB can cramp win7.
Thanks for the quick reply, I had planned to go with an SSD (two of them actually) for, as you said the OS and important apps. I just wanted to squeeze every last ounce of possible power out of my larger drives. I had planned to go with two of these: http://www.newegg.ca/Product/Product.aspx?Item=N82E1682... in RAID 0 or possibly 2 120GB SSD's, depending on how much space I need.