Mechanical hard drives aren't really pushing SATA2 and won't benefit from SATA3. In most single user environments even SATA1 would be enough. SSD's on the other hand are a different story, but they are not cost effective yet. Let's see again in a year or two.
Back to the topic. Windows 7 does indeed deserve to be bought, it's that great an OS. For a Windows. I've been using RC1 since it was released to public. Just for the kicks me and my friend at work decided to try how little RAM Windows 7 RC1 needs to start without errors. Unmodified Windows 7 RC1 booted and worked with as little as 96MB RAM. Yes, you read that right, 96MB RAM. With 64MB RAM it wouldn't boot, but 96MB is very impressive already.