Yes the newer stuff is SATA III.
In terms of performance over what you'd get at Dell or HP, you need to understand two things. First, is that the quality of components you get by building the system yourself over what HP//DELL slap together is night and day. You're getting a much higher quality system using a build like we've outlined. Second, HP and DELL use the low end CPUs from each class of processors in their systems, as opposed to a higher end chip like the i5 2500 that I spec'd for you. So performance-wise, you can expect an equivalently priced system from them to perform about 20% slower than what you're building here. That's just an average, however.
If you are up to putting the PC together yourself, it's a very rewarding feeling once you have it done. It's literally a paint by numbers kind of activity. On the other hand, if messing with hardware and wires sends you running in the other direction, it's best to hand a spec like weve put together to someone like cyberpowerpc, and let them build it for you.