Hi there monzoid,
Sorry that you are facing some issues with your WD drive.
As you believe that there's is something wrong with it, it would be a really good idea to back up the data stored on it. After that, you can run both and extended DLG tests, so you can see whether and what is wrong with it:
http://products.wdc.com/support/kb.ashx?id=bkon4s
Apart from that, if your current HDD is in a healthy state, then you will not really experience noticeable performance difference. Generally speaking, drives with bigger capacity are faster than smaller ones. (if we assume they have same RPM, just like in your case) Yet, it wouldn't be that noticeable.
If your current HDD has poor performance because of the fact that it is failing, then yes, you will get shorted boot and loading times with the new HDD.
Cheers,
D_Know_WD