well, in regards to your partition size, your formatted size of your drive will come down to approx 300G, So if you are going to be storing a lot of stuff i would maybe go for a 100G for XP and a 200G ( or the remaining space ) for your storage. You could even bring the XP partition down to 80G if you like.
I would use NTFS formatt.
What I would be inclined to do is if your current XP installation is running fine I would install your new hard drive in your system along side your old drive, use a partitioning tool such as
PARTITION LOGIC to partition and formatt your drive then use a program such as
XXCLONE to clone your old drive over to the new XP partition you setup, you will have to set an active partition on the new XP drive and then with xxclone make sure you make the clone bootable. this will then allow you to copy the whole contents accross and then go into the bios and change your boot order to boot to the new drive, once you are happy it is running fine you can either keep the old drive as a backup drive or formatt it and use it for whatever you like
have a look at this post by Linderman that explains how to clone your drive
HERE