Advice & Set Up of FreeNAS Software

I have built a NAS from scratch and wish to run FreeNas on the box but need some advice on the best way to config up the box. As I am using the Western Digital WD20EARS 2TB hard drives, there is an issue with the read difference of 512kb to the 4096kb settings now used in the new drives. This means that the read/write speed of the drives is prohibitively slow. I wish to set up some sort of mirroring raid (Raid 1 etc) as I am lead to understand that although this uses up double the disks, it is the best raid for recover and performance. I will be streaming HD video content from the NAS as well as storing media files etc. The spec of the NAS is as follows: Fractical Design R3 Case Gigabyte GA-P35C-DS3R (Vs 2) Intel Core 2 Duo E8400 2.8Ghz LGA775 Processor nVida GeForce 9300 GE PCI-E Dual Graphics Card 6 x WD20EARS HDD Drives 2 X 2GB DDR3 Ram 1 x 1Gb Transcend Compact Flash Card (Using IDE Adapter Straight Into Mother Board For OS) FreeNas 0.7.2 loaded onto compact flash card and installed. I require someone to discuss and advise me on the best config set up for the box. I have installed the box with FreeNas and played around with different set ups but just can't seem to get a decent I/O from the box. With Software raid it's literally kb/s when coping data via gigabit network and if I set up ZFS it increases but not by a lot. Not a huge project but just need someone that is knowledgeable about this. Can provide remote access to the FreeNas interface for set up. Pretty urgent project. Don’t mind installing a different version of FreeNAS on the box eg the Nightly one if this may work better but not prepared to put Vs 8 on there as too many bugs and issues and need a stable version running. Thanks loads!