if your going to install in new HDD just disconnect old HDD & continue with new one .. partition size depends upon your requirement..
may be 200 or 250 Gb for each one..
i suggest to install OS on new drive & move the virtual memory to the old drive after installing OS..
or just install OS on old drive (move the data to new one before formattiong old HDD)
and use new 500 gb drive for data