Hard Drives

Not sure if I could ask this here cause its has nothing to do with overlocking but here it goes.

PowerMac G5 Dual 2Ghz and it had 160GB hard drive so I replaced it with a 1.5TB and now I have a question.
Right now I have 1.7TB of hard drive, the 1.5TB is partition into 2 partitions 1000GB for Leopard and 500GB for the time machine I also have a 200GB for storage. The 500GB is almost full there like 40GB free so I want to upgrade with more storage but not sure on what to do. I plan on removing the 200GB when upgrading.

I could get a second 1.5TB and have 1.5TB for the OS and 1.5TB for time machine

or

I could raid the two 1.5TB for a total of 3TB and get a external hard drive for the time machine. I was thinking of get a external enclosure with two 3.5" bays to hold two hard drive.

Would raiding the two main drive and keeping the external for time machine be my best option? Should the External drive be raided as well and what would I need for the time machine two 1TB or two 1.5TB?

EDIT: out of the 1TB for the OS I have around 3XXGB left another reason I want to upgrade my drive to more space. Also how often does your time machine back up? I have it set to every hours I think.

EDIT: I just said raid 1.5TB cause I already have one but they can be smaller drive as well.