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#11
The tutorial that mate42nd posted will be the largest help once you review that. You can easily shrink the XP primary to see a new second one created or elect to see a stand alone install of 7 on the new drive and later copy the XP boot files over to it along with using a free program available to editing the 7 BCD in order to add the XP entry in.
If you plug the new drive in as the first sata and install 7 while the XP drive is still plugged in chances are that the installer will take care of seeing that done simply from detection of XP. Dual booting across drives allows more drive space for each installation if you need the local drive space. If not simply repartition the XP drive and use the new drive for primary storage.
The image here shows how a triple boot of both RCs and Vista turned out.