Multiple disks, across different physical servers (instead of RAID) would be good too. You could also try increasing the throughput by having an in memory cach

Multiple disks, across different physical servers (instead of RAID) would be good too. You could also try increasing the throughput by having an in memory cache. The index/hash table can also be in main memory. Depending on the keys, locality can play a crucial role in prefetching data.