What you need to do first is to slave the new drive to the old one. When you buy the new one get the shop to adjust the jumper to the slasve position and suply the cable to connect the two drives together. Also ask for guidlines on instalation and formatting, th new drive will need to be formatted before use.
Once installed and you know that it works, get your system disk and put the os on to the new drive. Once that is done install all programs that you presently use to the new drive and then simply transfer data files from one drive to the other. When you are happy that all is well, reconfigure the new drive to Master and remove the old drive. All should be well; it sounds complicated but isn't really, you just need to be organised and it does take some time to do.
The important part is having everything to hand and take your time in doing things, that way you will not lose any vital data, backup imprortant data files first though if you have not done so already!
Hope this helps.