It is always best to use identical drives for a RAID. Especially a software RAID.
I strongly suggest you NOT use RAID 5 if you are using your motherboard as the RAID controller. RAID 5 is very CPU intensive without dedicated hardware to handle it.
Please reconsider this. A RAID 10 or a 5 with a hardware controller card would be well worth the trouble.
This card is awesome, but expensive. If I ever find a winning lottery ticket I will buy one of these and eight drives to use with it in a RAID 6.
http://www.newegg.com/Product/Product.aspx?Item=N82E16816131004
Also learn a bit about TLER, Time Limited Error Recovery. You can enable TLER in your WD drives and that will greatly improve their reliability in your RAID.
http://en.wikipedia.org/wiki/Time-Limited_Error_Recovery
The utility referred to in the Wikipedia article can be found if you search for it.