How to make update rapidly?

Hi,Every body;
I have a table contains 100,000 rows, and has a primary key(int).
Now ,I need to execute sql command like "update .......... where id=*"(id
is primary key).
I expect execute 1200-1600 sqlcommands per second(1200-1600/s).
In test,when the id increase by degrees in sqlcommands, then I can reach
the speed(1600/s);
But in fact , the id in sqlcommands is out of rule, then the speed is
very slow, just 100/s.
what can i do? can you help me ?