Most modern HDD's are fast enough that there is little speed advantage in RAID0. Some gamers still think that is enough advantage to be worthwhile, though. But you must remember the risk of RAID0 - BOTH drives MUST function at all times, because the info is distributed over both. So a failure of one means ALL of your data is gone from the RAID0 array. That's double the risk of drive failure, compared to risk for a single drive.
Personally, I'd prefer two independent 500 GB drives over one 1TB drive. But, if you have the money and interest to buy a 1TB unit, why not add that as the second drive, instead of adding a 500 GB? If you are going to keep them separate the way I would, there's no problem doing that.