How will XP Pro help with a full drive? If you decide to get XP Pro, make sure to order a
OEM version, rather than waste a ton of money on a
retail box. There is over 100% difference in price between them..
If you want to use the second drive as a backup, look into setting up a
RAID 1 array with them (if your motherboard can support it, anyways). All data will be automatically mirrored to both drives, as it is written. That way, if one drive fails, all of your data is still on the other. You also get some minor read I/O performance gain.
(naturally a RAID 5 array would be much better, but you need at least 3 hdds to set one up)