If the 645's are anything like the 605's (and I assume they are) they are junk. The 605 servos center lousy and have ZERO holding power. I don't care what the specs are, you can take a 3D arm on a 605 and EASILY push it to the side against the servo, and I run 6V.

If you want to go cheap get at LEAST the 5645's instead, they still don't hold or center great but are MUCH better than their non-digital counterparts.

The other servo mentioned would be better but at twice the price.