I was hoping to set my two hard drives up with raid 0 this week. They are both 1TB WD Black drives, however one is 3Gb/s and the other is 6Gb/s. I have a Gigabyte GA-P55A-UD3 board with two 6Gb/s SATA ports on it. Would setting up Raid for my 3Gb/s and 6Gb/s drive bring down the speed of the 6Gb/s drive? Would it be a smarter idea to not raid and use the 6Gb/s drive by itself to maximize speed. Thanks for your help!
Even 1,5Gbps won't be a bottleneck; your HDDs cannot push more data. SATA 3Gbps means twice the theoretical throughput; SATA 6Gbps surely won't give you more performance (aside from propagation delay but that's minor).
In fact, you may want to connect both HDDs to your Intel 3Gbps controller and use RAID0 there, since it's faster than the fakeRAID your onboard 6Gbps Marvell controller can give you.
Even SSDs don't benefit from SATA 6Gbps at this moment, so don't assume HDDs do.