The fastest sata drives are ssd's (crucial, ocz, intel). Look for SLC based drives, MLC ones have lower life expectacy. Try to get 6 of them and strip them (raid 0).
If you cannot use ssd's, of find them expensive, you could add an sas raid card to the server ( i recommend areca), get some new seagate 15k.7 drives (200mbbyes/sec on sequantial read) and short stroke & strip them toghether.
All that if routeros can work with the way the raid card presents the array.
If routeros cannot see thoose arrays then 1 slc based ssd if your best option.
For more ssd info, you can read anandtech's articles.
good luck & good business!