I can give much better help over the phone but will post what I can.

Can you move all your data around so that one of your 300GB HD's is blank? I would recommend making a complete backup to the drive that is bootable. This is mostly only needed if you use IE, Outlook or Outlook Express as it is much easier to get your data moved to a new install if you still have it working. I would use Acronis True Image but it costs money. Here is a free program I've heard works DriveImage XML but I haven't tried it.

XP can install to SATA hard drives but it needs the drivers. I use tools to slipstream drivers and the latest service pack into one CD. This helps lower the total install time as your not spending hours installing updates. You can find out more about slipstreaming here.

When installing XP it will get to a spot that shows all of your drives and will ask where do you want to install XP. This is where you would delete the current partition on your 75 GB drive. You would then create a new partition and format NTFS quick. You can pick the NTFS normal but it really isn't needed.