Performance-wise, three drives in RAID 0 will be faster.
(I may be inviting a huge debate by making such a bald statement, but I believe it.)
In terms of reliability, three drives in RAID 0 are much more of a risk. If one drive dies, the other two are useless. With three drives, the chance of a failure in any given period of time very close to triples. And if the mobo fails, it can be very difficult to read the data off the drives even if they are in perfect condition.
So: get both the 3 500 GB drives and the 1.5 TB drive. Put the 1.5 TB drive in a removable bay, and do frequent backups. Take the drive out when not backing up, and keep it in, at least, a separate room.
absolutely... and the more HD's you add.. the more performance goes up, as well as you don't lose space. so totally worth it, as long as you remember that, if only 1 drive fails in RAID 0 -> you lose all the data