I'm looking at building a new PC sometime(currently trying to justify spending the money, lowering the cost would make it easier) and one of the options I was looking at was getting a mobo and HDD with SATA at 6.0 Gb/s. What I'm wondering though is...does going from 3.0 Gb/s to 6.0 Gb/s make a difference if you're on a HDD anyways(I would think that there's some limitation inherent with the drive being mechanical)? I can definitely see it being worth it on a SSD but the only SSDs at SATA3 on newegg are in excess of 300 dollars and way larger than I would need(If I got a SSD I'd only want it for Windows and one or two games at a time. But I'm going to wait for the prices to drop before I ever consider a SSD).
I'm talking about this from a gaming and boot time stand point(not after crazy SSD 7 second boot times, I'm just curious if there's actually an advantage over SATA2 here). I don't use that much in the way of heavy apps.
The main thing is that most SATA 3 drives have huge caches, which arguably create the perfrmance gains seen in benchmarking vs the actual difference in the interface (SATA 2 vs SATA 3). Also, if you plan to use 6GB/sec interface with RAID, make sure your motherboards or PCIe SATA 3 controller supports it. Some motherboard support RAID on the SATA 2 controller but not the SATA 3 controller.
For hard drives that are otherwise identical SATA 6.0Gbit/sec has no significant advantage over 3.0Gbit/sec. The drive itself can't accept or deliver data that quickly, so there really isn't any benefit to the higher speed.