I am bying a mobo with several Sata-3 connectors (the ones with 6 GB/s transfer rate). As I'm buying a new system, I thought it would be better to buy a new Sata 3 hard drive - they work mush faster and generally cost the same. Should I buy one? And where?
SATA Version 3.0 hard drives DO NOT perform any better than SATA Version 2.0 hard drives. The SATA interconnect may be faster, but the hard drive itself only spins so fast (typically 5400 or 7200 rpm), and THAT's what limits the maximum transfer rate to or from the disk. It's only extremely expensive 15,000 rpm Enterprise-class drives that are currently getting close to the limits of SATA Version 2.0.
Even most SSDs don't take full advantage of SATA 2.0.
To make an analogy - a bicycle isn't going to travel any faster just because you raise the speed limit from 30MPH to 60MPH. The cyclist is the limiting factor, not the posted speed.
So if you're buying a hard drive, look at it's general performance specs (transfer rate, access time) and don't worry about the interconnect - it won't make any difference.