Welcome aboard! Hopefully you'll find that you don't have to worry too much about questions you have when trying something new. We'll help as much as we can. :)
The ATA rating is nothing more than the maximum theoretical throughput that you can expect to see. It is unlikely that you'll ever get in this range. That said, all of the ATA133 controllers are backwards compatible with earlier technologies. You'll be safe in this regard.
As for the large buffer, I can't say for sure because I've not used two WD drives in a RAID, but there should be no problems with it. The buffer is nothing more than a means holding extra data that you'll need soon so that it can have it ready to transfer to the system faster than searching for it on the drive platters. The benefit of a RAID 0 setup is that it can do this with two drives at the same time. I can't think of a reason that it would conflict with the array.