Hi there Heshai,
It's great you've decided to go with WD Black.
Yet, I believe that all WD Black models come with 7200 RPM.
Anyway, you could consider cloning the drive as well. This is a process that would make the exact same copy of your OS drive and put it on the new WD Black. You can do so with Acronis True Image WD Edition:
http://products.wdc.com/support/kb.ashx?id=DSm3QT
You will need to attach the new drive to the system, perform the cloning process, take out the old drive and plug the new one on it's place.
If you get poor performance, then this would be because of the fact that your old drive was failing.(creates exactly the same copy) In that case, you will need to perform a clean OS installation.
Cheers,
D_Know_WD