From what I have read the Mac Pro does not support RAID 5 without a RAID card. Which can be quite expensive. The only RAID card I know of the works with internal Hard Drives on the Mac Pro is from Apple. Other Manufacturers cards require external drives.
RAID 5 does provide decent protection, but your setup should be good enough as long as you backup to your 1TB drive regularly. Then the most you lose is your days work, work between backups. With how few drives you have you could do RAID 0+1 which would not require an additional card, but you would need two internal and two external via FW800 for RAID 0+1.
Honestly the plan you laid out sounds fine this is a lot of storage and will really cut down on latencies. Unless of course you are using 512MB of RAM

1. 150GB 10000 RPM
2/3. 640GB's RAID 0
4. 1TB: Time Machine/Superduper
Plus with a backup if you accidently delete a file you can recover it as opposed to RAID 5.
The other setup you could do though not quite as efficient would be.
1-4. 640GBs in RAID 0+1 for boot, data and apps
5. External 1TB for time machine
But I would go with the first option. It really depends on how important efficiency and real time data protection is for you and their are many different configurations. The biggest factor is how much are you willing to spend; then comes the importance of speed versus real time data security versus amount of storage.
Edit: You don't want to go too nuts.