RAID-1 will give you no performance increase for writes, and may give you some performance increase for reads. However it will give you redundancy, which you probably want anyway.
RAID-10 will give you approximately 2x performance increase, and will give you redundancy too. So this is what you most probably want.
As for 15k drives - yes, they are better, however quite more expensive. You have to decide here - how fast is fast enough and how much you are willing to spend to go even faster.
40 GB is not that much. Have you considered a SSD? It would be many times faster than any hard drive.
Just keep in mind that even though SSD has no moving parts it can still fail and loose your data. So you either have to make RAID1, or make regular backups (perhaps dump the entire database to a HDD every hour). It all depends on how important is the uptime and data of this database and how much you are willing to spend.