1) More than likely that 150's will be ok
2) Possibly, that is the main reason for PCI-X and alot are also using PCI-e now... is there any chance that your current machine has PCI-e?
3) On a seperate RAID card like this you will not have an issue, processing will be done onboard and will more than likely not cost you an CPU time at all.
4) actually found this on the data sheet "Storage capacity up to 4TB (4x500gb hard drives in 2x controller system) So you would be fine with that limitation. Let it be known though that anything over 2TB partition will not be recognised with "normal" master record. You would need to convert to GUID table, which is supported in 2k3
Personally id go for WD or Seagate 500gb drives they are best $/Gb value atm and cheaper for not much performance lost, and in sequential read/writes with large files (CAD drawings) the bigger WD or seagates will be faster than the Raptors. I would also look at getting a PCI-e RAID card if you have a PCI-e port as these will be quicker and will also allow you to grab an 8port card for scalibility in the future. I currently own an Adaptec 3805 which is awesome, a little expensive but awesome.
Hope i helped.